Remove `source_files` from top-level podspec

Because of that line all source files were added to the project when you specify `pod 'TransformerKit'` and `pod spec lint` was failing because `Security` framework is needed for `TransformerKit/Data`, but wasn't added as dependency. Looks like now developers need to specify what parts (subspecs) of the library do they need. For example in my case `pod 'TransformerKit/String'` works well.

This change has been already merged into CocoaPods/Specs#11113