infra: Remove 'win_toolchain' cache from all builds

The "win_toolchain" cache is a legacy cache kept around from a long
time ago. It's unclear what purpose it serves/served.

This cache is currently requested on builds for many (100s) chromium
builders, and it was always unclear if the builds actually used the
cache.

But now that swarming deletes all empty caches from bots, we can
check how many bots actually have one or more files present in that
cache. ie: Any bot that advertises the cache in its dimensions is
guaranteed to be using said cache:
https://chromium-swarm.appspot.com/botlist?c=id&c=task&c=os&c=status&d=asc&f=caches%3Awin_toolchain&k=caches&s=id

From that list, _only_ bots in infra flex pools are actually persisting
files in the win_toolchain cache. This means that all chromium bots
do not. And so we can stop requesting that cache on chromium builds.

Bug: None
Change-Id: I84687b4b6fecbae564dd0a4ac55ce0d1596306cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4276061
Reviewed-by: Struan Shrimpton <sshrimp@google.com>
Reviewed-by: Garrett Beaty <gbeaty@google.com>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1109054}