perl: revert "re-enable the postPatch hook on Darwin"

This reverts commit 1778200519a18ae00751ab3015d0e665373c32f6.
Apparently, the sed -i syntax is different from what I thought, because
now the build fails on Darwin, saying:

sed: -i.bak: No such file or directory

See http://hydra.nixos.org/build/5829944/nixlog/1/raw for more details.

pkgs/development/interpreters/perl/5.16/default.nix

···
     cpan/CPANPLUS/t/20_CPANPLUS-Dist-MM.t \
   '' + " ";
 
-  postPatch = ''
+  postPatch = optionalString (!stdenv.isDarwin) /* this failed on Darwin, no idea why */ ''
     for test in ${testsToSkip}; do
       echo "Removing test" $test
       rm "$test"
       pat=`echo "$test" | sed 's,/,\\\\/,g'` # just escape slashes
-      sed "/^$pat/d" -i.bak MANIFEST
+      sed "/^$pat/d" -i MANIFEST
     done
   '';