Remove proxy modes from SpawnedTestServer.

Now that nothing is using them, best to remove the API to prevent
regressions. This is not intended to remove every scrap of proxy related
or more generally unused code from the SpawnedTestServer, since we'll
(hopefully) be able to delete it soon anyways, but rather as a quick
minimal-effort feature removal.

Fixed: 40242862
Bug: 40419904
Change-Id: Id4ab128912a1d52c0a9e72ac39a9bb70bbf8e8e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6674254
Reviewed-by: Adam Rice <ricea@chromium.org>
Commit-Queue: mmenke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1479245}
NOKEYCHECK=True
GitOrigin-RevId: 4a531243ca7b1062c78aeead9cc818714e4d46eb