Test suite: drop Seahorse tests

These tests are the only part of our test suite that depends on v2 Onion Services, which already prevents running our test suite when the developer uses a recent tor.

Given we decided to stop investing much time into the "OpenPGP outside of Thunderbird" area, in order to focus on the needs of our main personas, let's remove these tests instead of porting them (which might be cheap, or not, but even investigating that would be a poor use of our resources IMO).

refs #18256 (closed)