Part of WT-18574. These changes are test only, the cache is soft-disabled (WT-15663), this stops anything requesting it.

test format changes:

• drop block_cache* from test/format/config.sh and regenerate format_config_def.c (no hand edits)
• remove the format_config.c consumers and the wts.c connection string
• strip block_cache* from the 56 CONFIG files (54 failure configs plus CONFIG.disagg and CONFIG.endian). Evergreen task arguments in the same PR.
• Compat: old format binaries still parse block_cache and new ones must warn-and-ignore on replay - re-check the compat task setup, there is no block_cache entry in config_compat.c today.

test suite changes:

• in test_layered_config02.py remove the FIXME-WT-15663 skip, the block_cache=(enabled=true,...) config and the block_cache_blocks_removed assertion
  • keep the long-delta-chain coverage on the default config. 
• Check stat-enumeration tests against the 22 legacy counters; the victim-cache counters already live under disagg stats (WT-18764), so the whole remaining BlockCacheStat group goes.

Definition of done:

• no block_cache* consumer remains in test/
• historical CONFIG files replay with only a warning, no src/ changes.
• Merges before WT-18874, which deletes the counter and closes WT-15663.