test: remove cross-chain parity upgrade fork test

The suite forked mainnet at latest (no pinned block) and asserted live
config values, which made it a time bomb: EthUpgradeForkTest's
test_postUpgrade_liquidatorHasReflowBlacklist asserted that the ETH
Liquidator's fundSource is address(0) on the reasoning that an upgrade
with empty data leaves it at the default. fundSource is storage, so
upgrading the implementation never resets it -- the assertion only held
while nobody had set it on mainnet.

It is now 0xC1D8F6072D8a987BCbB675dc4805E8A2dE604899 on-chain, so the
test failed on every PR and on master regardless of the change under
test.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: razww