Update install.py to support removing old extensions

Gemini cli no longer allows _ in extension names. This prevents it from
deleting extensions that already exist. Instead of passing these to
gemini cli to get rejected just attempt to delete the extension from
the extensions dir

Bug: 448417182
Change-Id: I711f5247cad607f666094639bc22b517e22927cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7000837
Commit-Queue: Struan Shrimpton <sshrimp@google.com>
Reviewed-by: Erik Staab <estaab@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1523143}