Clean up Xcode optimizer settings and defaults/blanks.
Clean up Xcode optimizer settings and defaults/blanks.

- Move the optimizer settings to the to the project level so they are the same
  between iOS and macOS.
- Remove settings that are blank/default values.

thomasvl/swift-protobuf,apple/swift-protobuf,tbkka/swift-protobuf,tbkka/swift-protobuf,cburrows/swift-protobuf,apple/swift-protobuf,thomasvl/swift-protobuf,allevato/swift-protobuf,cburrows/swift-protobuf,tbkka/swift-protobuf,cburrows/swift-protobuf,thomasvl/swift-protobuf,allevato/swift-protobuf,allevato/swift-protobuf,allevato/swift-protobuf,tbkka/swift-protobuf,apple/swift-protobuf,thomasvl/swift-protobuf,cburrows/swift-protobuf,allevato/swift-protobuf,allevato/swift-protobuf,apple/swift-protobuf,apple/swift-protobuf,cburrows/swift-protobuf,cburrows/swift-protobuf,allevato/swift-protobuf,thomasvl/swift-protobuf,thomasvl/swift-protobuf,tbkka/swift-protobuf,thomasvl/swift-protobuf,tbkka/swift-protobuf,apple/swift-protobuf,apple/swift-protobuf,tbkka/swift-protobuf,cburrows/swift-protobuf