Correctly deprecate -mtl_dictionaryByRemovingEntriesWithKeys:

As per https://github.com/octokit/octokit.objc/pull/184, this restores and deprecates the old `-mtl_dictionaryByRemovingEntriesWithKeys:` method for sets and instead adds a version for arrays with an slightly different name.
This can just be `-Wdeprecated`, FWIW.