firebase-ios-sdk

Clean up Objective-C remnants (#3866)

* Remove references to removed classes

* Remove references to FSTSyncEngine

* Fix references to Objective-C names that are no longer valid.

* Remove forward declarations to classes that don't exist

* Remove FSTTypes.h

* Remove usages of headers that no longer apply

* Clean up header imports

* Remove traces of Objective-C from api/firestore.{h,mm}

* Strip NS_ASSUME_NONNULL directives that don't apply

* Remove FSTSyncEngine.h