Remove env config fetcher code

This code is no longer needed since we are now setting USE_RBE to true
for 100% of users (except two users, who I'll reach out to over email).
So fetching the configuration is no longer needed since sometimes this
can result in RBE config flipping between on and off (depending on
whether you have gcert or not).

Tested: Ran the build with latest sync and ensured RBE was used for the
build.

Bug: b/283828386
Change-Id: I3cda78607b46a0e161b7841560dc587a44d91f27