Remove dead KernelTestBase::stop() method

Seems pretty straight forward.

Committed/pushed to 11.x, thanks!

We should have a 10.3.x MR to deprecate it with no replacement in 10.3.x, but since this is blocking phpunit 10 and also dead code that no-one's likely to call, went ahead with the 11.x commit without that being in place.

catch → closed merge request !7831

Deprecation for 10.3

10.3 MR seems good too.

Committed the deprecation to 10.3.x and published the CR. Thanks!

xjm → closed merge request !7835

Automatically closed - issue fixed for 2 weeks with no activity.