Part of WT-18574. Covers the deprecation and the strip.

Deprecation:

• block_cache.enabled=true returns ENOTSUP when it is set in the API string, WiredTiger.config or the environment (open and reconfigure); explicit settings in any source warn once under WT_VERB_CONFIGURATION and are ignored; the category keeps parsing. An existing config string or persisted basecfg with block_cache=(...) must still open - unknown config keys are EINVAL (config_check.c), so the category can't just leave the grammar. 
  • Per-source check beside every _conn_chunk_cache_check call site (API string, WiredTiger.config); the WIREDTIGER_CONFIG check goes after the final stack build, not inside _conn_config_env (runs twice, double-warns). The base configuration file is machine-written: warn only, never reject - a persisted enabled=true opens, the user should not have to override it.
  • Add every block_cache subkey to __conn_write_base_config's strip list.

Stripping it out:

• remove the get/put paths and cache-hit branches from the block_io.c read/write paths and the remove calls from the free paths
• delete the destroy path: the function, its helpers, the teardown call in conn_open.c 
• delete the now-call-less functions, the orphaned setup statics and all 22 counters (counters drop with their last updater, s_stat fails otherwise).
• Remove the FIXME-WT-15663 soft-disable.

Definition of done:

• open/reconfigure regressions cover the contract
• encrypted+compressed / corruption / checkpoint / disagg base+delta tests pass
• strict builds and dist/s_all pass alone.
• Closes WT-15663 as Won't Fix.
• Merges after WT-18872, before WT-18876.