[rust/coverage] Remove obsolete WAR to pass -z,start-stop-gc

Bug: http://b/199203503

Rust's llvm-project source now includes  https://reviews.llvm.org/D97448
and we no longer need this workaround.

Test: m unicode-xid_test_src_lib and verify __llvm_prf_data section is
      present.
Change-Id: I0387bd3af8bfc765f3aba3b309bfbc7ef49d565d