Don't validate vendored_libraries path

This removes the validation for vendored libraries to be identified with `foo` instead of `libfoo.a`. Since `libfoo.a` is the library's file path it makes sense to allow that directly. Invalid libraries will still be caught with https://github.com/CocoaPods/CocoaPods/blob/0dc6fa95b3636fa5923a401e2fea52e2f40c0e1e/lib/cocoapods/validator.rb#L293