Remove SpawnedTestServer.

It's deprecated, and tests have all been migrated off it.

This CL also enables websocket_end_to_end_test.cc on iOS, which
previously wasn't built there due to SpawnedTestServer not working on
that platform.

This leaves net/data/websocket in place, other than the Python files
in that directory, but I plan to remove it and the
`serve_websocket_test_data` argument to InstallDefaultWebSocketHandlers
in a followup CL, moving files to the relevant chrome and content test
data directories as needed, just like non-WS BrowserTests do.

Fixed: 40419904
Change-Id: Iab71a9ce79256898affc036c7c450c512fe44f48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6713638
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Commit-Queue: mmenke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1484928}
NOKEYCHECK=True
GitOrigin-RevId: eab727f8ba864cc2e7aa4fddd51e532a4ce0b2d4