Remove a Darwin hack that should no longer be needed

src/libstore/local-store.cc

@@ -964,13 +964,6 @@ void LocalStore::startSubstituter(const Path & substituter, RunningSubstituter &
 
     case 0: /* child */
         try {
-            /* Hack to let "make check" succeed on Darwin.  The
-               libtool wrapper script sets DYLD_LIBRARY_PATH to our
-               libutil (among others), but Perl also depends on a
-               library named libutil.  As a result, substituters
-               written in Perl (i.e. all of them) fail. */
-            unsetenv("DYLD_LIBRARY_PATH");
-
             /* Pass configuration options (including those overriden
                with --option) to the substituter. */
             setenv("_NIX_OPTIONS", settings.pack().c_str(), 1);