Clean up the flag enable_rlp_callbacks_in_media_router2

The flag has been fully launched in Android V. We need to clean up
the flag usage from the codebase. But we should keep the flag
declaration because the flag is an exported flag.

Bug: b/409609485
Fix: b/409609485
Flag: EXEMPT removing com.android.media.flags.enable_rlp_callbacks_in_media_router2
Test: presubmit
FCRS_CODE: m9pi0jf7o9v2l2
Change-Id: Ibed2eb9028d14c76d8f030a5fc423a335abf94f1