Firestore: Remove deprecated APIs

[paulb777]

Please remove deprecations in public headers in Firebase 7

~/gh4/firebase-ios-sdk/Firestore/Source/Public/FirebaseFirestore (master) $ git grep -B5 -i deprecat
FIRFirestoreSettings.h: * @deprecated This setting now defaults to true and will be removed in a future release. If you are
FIRFirestoreSettings.h- * already setting it to true, just remove the setting. If you are setting it to false, you should
FIRFirestoreSettings.h- * update your code to expect FIRTimestamp objects instead of NSDate and then remove the setting.
FIRFirestoreSettings.h- */
FIRFirestoreSettings.h-@property(nonatomic, getter=areTimestampsInSnapshotsEnabled) BOOL timestampsInSnapshotsEnabled
FIRFirestoreSettings.h:    __attribute__((deprecated));