Fix distclean for "pm_to_blib directly" extensions.

The values of the %pm hash already have ../../ prepended, so we
were trying to remove, for example, ../../../../lib/Exporter.pm
instead of ../../lib/Exporter.pm.

This fixes [perl #122820] wherein it was reported that a distclean
make left some build products under lib/.

make_ext.pl

@@ -719,7 +719,7 @@ sub just_pm_to_blib {
             # (which it has to deal with, as cpan/foo/bar creates
             # lib/auto/foo/bar, but the EU::MM rule will only
             # rmdir lib/auto/foo/bar, leaving lib/auto/foo
-            _unlink("../../$_")
+            _unlink($_)
                 foreach sort values %pm;
         }
     }