Delete symlink of non-existent destination in revoking

In a phase of revoking CreateLink, it confirms the existence of file only
by `File.exists?`. So when the destination is symlink and the source file does
not exist, it returns false. This can be trouble depending on the execution
sequence.
Merge pull request #297 from r7kamura/revoke_symlink

Delete symlink of non-existent source in revoking