cronet: delete option to disable cronet in tm-mainline-prod

With the release cut of the July train and the recent automerger
cutover, tm-mainline-prod is now officially an abandoned branch.

This change deletes (most) infrastructure that was put in place to
disable cronet on tm-mainline-prod.

Test: builds
Change-Id: I078f2114b736a634f08d8f704c19beb2224ef645