All other browsers use a single configuration for their browser
invocations on WPT. Servo historically had two: servo and
servodriver. Now that we run WPT on Servo GitHub CI with
Webdriver using the servodriver, we can align our configuration
with theirs.

To do so, we drop most of the custom implementation. The existing
WPT configuration for the Servo binary duplicates some work in
the executor that the browser configuration does instead. Therefore,
we now configure preferences and such in the browser configuration
rather than the executor.

We also use the existing implementation for the servodriver
configuration for the servo browser. The reason that we don't
switch to servodriver always is that on wpt.fyi we use the Servo
product and that's the one that also daily runs. Since we don't run
servodriver externally and it isn't intended either, I think it's
better to configure Servo probably.

This does mean that in a follow-up PR we can remove the full
servodriver WPT configuration. However, to make sure the existing
setup works, we do that first and delete the servodriver WPT
configuration once we are confident everything is stable.

All in all, this ensures that both on local, Servo GitHub CI and
on wpt.fyi we all use the exact same configuration. I tested
this locally by running the following test:

```
./mach test-wpt tests/wpt/tests/css/css-overflow/scrollbar-gutter-dynamic-004.html
```

This does times out with the servo binary and works with the
servodriver binary.

Fixes web-platform-tests#40751

Signed-off-by: Tim van der Lippe <[email protected]>