Revert "Use arrays when removing dictionary entries"

This reverts commit 502175f690e14d30c09506fc69d957a79ab0d3b9.

Conflicts:
	MantleTests/MTLTestModel.m
Add mtl_dictionaryByRemovingValuesForKeys:
Fix copypasta :spaghetti:
Ignore -Wdeprecated instead
:unamused:
Merge pull request #338 from Mantle/removing-keys

Correctly deprecate -mtl_dictionaryByRemovingEntriesWithKeys: