Remove cocoapod's post-install header rewrite

[dennycd]

At the moment, post pod installation, all gRPC/ObjcC pod is using prepare_command to rewrite c++ core / external lib's include headers in order to accomodate iOS/Xcode search path. This creates various issues and very error prone.

Instead we should look at avoid modifying the source and adjust build header search path at Cocoapoad/Xcode configuration level.