Fix Filesystem::executeRemovePath

Summary:
Filesystem::remove() (implemented by executeRemovePath()) was broken for
symlinks to directories because is_dir returns true for symlinks to directories
in php.

Test Plan:
Tested in hphpd that is_dir() returns true for symlinks to directories, and that
unlink() instead of rmdir() will remove them (rmdir() won't remove them).
I haven't tested this code specifically.

Reviewers: epriestley, vrana

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4861