Deduplicates the various `prpc call` subprocess.run calls throughout
mcp/ in favor of a new run_prpc_call helper.
Also makes a few drive-by cleanup changes that were caught by WIP
presubmit checks.
Change-Id: I3e401cb959ba96f2224ea1a7f7f58795330f643a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6847120
Commit-Queue: Struan Shrimpton <sshrimp@google.com>
Auto-Submit: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Struan Shrimpton <sshrimp@google.com>
Brian Sheedy