Don't check whether gnuplot is present in $PATH, because we know it is.

diff --git a/eazy-gnuplot.asd b/eazy-gnuplot.asd
index 7b1c09b..4657cd6 100644
--- a/eazy-gnuplot.asd
+++ b/eazy-gnuplot.asd
@@ -37,7 +37,4 @@
                                :fill-pointer t)))
           (setf (fill-pointer seq) (read-sequence seq stream))
           seq)))
-  :perform (load-op :before (op c)
-                    (unless (zerop (nth-value 2 (uiop:run-program "which gnuplot" :ignore-error-status t)))
-                      (warn "Could not find GNUPLOT in $PATH")))
   :in-order-to ((test-op (test-op eazy-gnuplot.test))))
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<div class='path'>path: <a href='/guix/log/?id=751d1f01e4f0607d41e4c859d944753b18466652'>root</a>/<a href='/guix/log/README?id=751d1f01e4f0607d41e4c859d944753b18466652'>README</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/README?id=751d1f01e4f0607d41e4c859d944753b18466652&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2019-11-09 23:04:14 +0100'>2019-11-09</span></td><td><a href='/guix/commit/README?id=0758a70cc008171f31c848128deca9803536fe60'>doc: Mention value /var to localstatedir option.</a><span class='msg-avail'>...</span></td><td>zimoun</td></tr>
<tr><td><span title='2019-09-04 23:41:07 +0200'>2019-09-04</span></td><td><a href='/guix/commit/README?id=f475c6e53e5b6912cc45663d50f1d03dadfb60b8'>doc: Update 'README'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-01-09 14:47:53 +0100'>2019-01-09</span></td><td><a href='/guix/commit/README?id=0991fd53367907908fbd901a9fbe79540e4e4527'>build: Require Guile 2.2.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-12-02 17:56:05 -0600'>2018-12-02</span></td><td><a href='/guix/commit/README?id=6776af04d4b65c5a9a07784307a6abe395b6f480'>Make Guile-JSON a required dependency.</a><span class='msg-avail'>...</span></td><td>Eric Bavier</td></tr>
<tr><td><span title='2018-11-23 15:42:00 +0100'>2018-11-23</span></td><td><a href='/guix/commit/README?id=60e1c1099fc3d73ed7d3235e71aae5d00ab7d773'>Update Guile-SQLite3 URL everywhere.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-09-04 17:25:11 +0200'>2018-09-04</span></td><td><a href='/guix/commit/README?id=ca719424455465fca4b872c371daf2a46de88b33'>Switch to Guile-Gcrypt.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-06-27 16:58:59 +0200'>2018-06-27</span></td><td><a href='/guix/commit/README?id=fc6cbb311c5a1acb9cf1d5422fbf1bc23da31d55'>doc: Specify Guile-SQLite3 minimum version.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
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<tr><td><span title='2018-02-26 18:19:34 +0100'>2018-02-26</span></td><td><a href='/guix/commit/README?id=1d84d7bf6052c0c80bd212d4524876576e9817d4'>build: Require Guile &gt;= 2.0.13.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-01-07 23:47:22 +0100'>2018-01-07</span></td><td><a href='/guix/commit/README?id=06e3a5181efa0ea83bb6608d3cbfba5caa56d7e9'>doc: Mark zlib as mandatory, libbz2 as optional.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-01-01 18:21:55 +0100'>2018-01-01</span></td><td><a href='/guix/commit/README?id=397cd9cc416c677e84fb4bb6efef5f99d1d2394f'>doc: Update requirements in 'README'.</a><span class='msg-avail'>...</span></td><td>Mathieu Lirzin</td></tr>
<tr><td><span title='2017-09-25 00:12:49 +0200'>2017-09-25</span></td><td><a href='/guix/commit/README?id=e8d8ecde04a988f04642d754eccc16a14e0cdc53'>README: Replace http:// with https:// where applicable.</a></td><td>Marius Bakke</td></tr>