Remove chunk cache support from the test/format randomised stress tester.

Changes needed:

• test/format/format_config_def.c — remove chunk_cache, chunk_cache.capacity, chunk_cache.chunk_size, chunk_cache.storage_path, chunk_cache.type config entries
• test/format/format_config.c — remove chunk cache config assembly logic
• test/format/wts.c — remove chunk cache from connection config construction

Acceptance criteria: test/format builds and runs without any chunk cache configuration.