Remove in-repo podspec

I appreciate the efforts of @chakrit and @mattyohe adding and maintaining this podspec, but it seems misleading to keep it in the repo if we won't be ensuring that it's maintained. :\

Since it's still available in the [Specs](https://github.com/CocoaPods/Specs/tree/master/Mantle) repo (and actually more up-to-date there), it seems reasonable to remove this and just direct people there.