[native] Removing gRPC C++ deleted files from the XCode project

Summary:
Removing of gRPC C++ already deleted files from the XCode project. This is a follow-up from D5706 where files were deleted.

Linear task: ENG-2276

Test Plan:

1. Open XCode project -> navigate to the "Project navigator" (folder icon). Expand all folders to show files. There are no red-marked files (missing files).
2. The iOS app is successfully built.

Reviewers: atul, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5766