Revert "Detect RBE builds and accelerate them."

This reverts commit f4fa0cc7c1.

Reason for revert: This broke chromecast infra

Original change's description:
> Detect RBE builds and accelerate them.
>
> Chromecast builds use a buildflag called `use_rbe` instead of
> `use_remoteexec`.
>
> Bug: None
> Test: run `autoninja` in the chromecast internal repo
> Change-Id: Iebb18ad47a30c4152923f48175222ec6a20e16f4
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4144469
> Reviewed-by: Gavin Mak <gavinmak@google.com>
> Auto-Submit: Simeon Anfinrud <sanfin@chromium.org>
> Commit-Queue: Gavin Mak <gavinmak@google.com>

Bug: None
Change-Id: Ib93820b6045088b7dcb45d1f14c0d4f83dc92ebc
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4157771
Commit-Queue: Gavin Mak <gavinmak@google.com>
Auto-Submit: Matt Swartwout <mwswartwout@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

autoninja.py

@@ -98,9 +98,6 @@ def main(args):
                      line_without_comment):
           use_remoteexec = True
           continue
-        if re.search(r'(^|\s)(use_rbe)\s*=\s*true($|\s)', line_without_comment):
-          use_remoteexec = True
-          continue
   else:
     for relative_path in [
         '',  # GN keeps them in the root of output_dir