[build] Remove unused toolchain update script

TBR=sergiyb@chromium.org

Bug: v8:6857
Change-Id: I2b02d3352e5b13b73ed74a76405e15c266593873
Reviewed-on: https://chromium-review.googlesource.com/690079
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48216}