Collapse the SSV_TEST_BOOLE_FORK CI matrix after the Boole fork finalizes

[momosh-ssv]

Follow-up to #2964 / #2974.

Once the Boole fork has activated and stabilized, and the pre-fork code paths are removed from the codebase (alan domain branch in DomainTypeAtSlot, pre-fork proposer formula, alan topic names, discovery next-domain acceptance, etc.), the two-leg fork test matrix stops earning its keep — both legs would exercise identical code.

At that point, in the same PR as the pre-fork code removal (so CI and code never disagree about which fork worlds exist):

• Flip the test network config's default to post-fork, or delete the SSV_TEST_BOOLE_FORK switch entirely once it loses its meaning.
• Delete the ssv-boole-post job from .github/workflows/unit-test.yml and spec-test-boole-post from spec-test.yml. Keep the original ssv / spec-test jobs — they own the Codecov upload and the required-status-check slots.
• Remove the now-dead pre-fork fixtures. Note TestBuildPeerTrimScores_ComputesScoresForAllCandidates_BooleTopics becomes redundant with its non-Boole sibling at that point, so one of the pair goes.
• If ssv-boole-post / spec-test-boole-post were added to branch-protection required checks in the meantime (see Enable the SSV_TEST_BOOLE_FORK=post unit-test matrix (deferred from #2941) #2964), remove them from that list first so merges don't get stuck waiting on deleted jobs.

Do not do this at fork activation — as long as pre-fork branches physically exist in the code, the pre-fork leg is the only coverage they have.