Remove enums from mongoc-flags.h that are not referred to in supported API:
 

• mongoc_reply_flags_t is unused.
• mongoc_delete_flags_t is only referred in the deprecated mongoc_collection_delete.

Rationale

There is likely no use-case for these enums.

Quoting from mongoc_query_flags_t: "These flags correspond to the MongoDB wire protocol". The flag enums correspond to the Legacy Opcodes.

MongoDB server 3.6 supports the OP_MSG Opcode. As of CDRIVER-4098, the C driver no longer supports servers older than 3.6.