daemon: Micro-optimize 'deletePath'....'remove' calls 'unlink' first and falls back to 'rmdir' upon EISDIR.
This change gets rid of the 'unlink' call for every directory being
removed.
* nix/libutil/util.cc (_deletePath): Call 'unlink' or 'rmdir' depending
on 'st.st_mode', rather than call 'remove'.