Remove deleting cxx files after build

Summary:
linear issue: ENG-2013
Deleting lib.rs.h, lib.rs.cc and cxx.h after build causes IDE to display errors, and to be unable to index our cxx functions.
These files are in .gitignore, so this should only change what is seen locally.

Test Plan: Build ios, stop ios, see that lib.rs.h, lib.rs.cc and cxx.h files are still present in native/native-rust_library/

Reviewers: varun

Reviewed By: varun

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5350