[cronet] Remove workaround for non-empty additional_target_cpus

The support for additional_target_cpus will soon be removed. The
last usage of this variable by the bots has been removed (it was
used by iOS WebView builder).

This mean that it is okay to remove the workaround.

Bug: 1337780
Change-Id: Ib07e62e64237994ffe4d59a15aacbb7bb7c0ac5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4341715


Reviewed-by: Richard Coles <torne@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1118227}