tests: Remove Selenium tests

tests: Remove Selenium tests

These were added quite some time ago in order to allow some level of UI
testing. However, I've personally never used them, they're not used by
the CI, and no one has shown any desire in extending them in their time
here. It is time to bid these tests adieu.

Removing these allows us to remove a whole load of wiring that existed
just to enable these. Some of this, like the '--quick-tox' option for
the Dockerfile, is retained so we don't need to use different commands
for various versions of Patchwork, but the majority is just stripped
out.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Cc: Daniel Axtens <dja@axtens.net>