libmongoc has several generic command helpers that are read/write agnostic.

• mongoc_client_command()
• mongoc_client_command_simple()
• mongoc_client_command_simple_with_server_id()
• mongoc_client_command_with_opts()
• mongoc_database_command()
• mongoc_database_command_simple()
• mongoc_database_command_with_opts()
• mongoc_collection_command()
• mongoc_collection_command_simple()
• mongoc_collection_command_with_opts()

These helpers should not be inspecting or validating options supplied by the user.

See this PR conversation for context: https://github.com/mongodb/mongo-c-driver/pull/662#discussion_r454365303

For example, most go through _mongoc_client_command_with_opts will call mongoc_cmd_parts_append_read_write to apply command options. In doing so, the server's wire version is checked for read/write concern support. This directly contradicts the read/write concern spec.

Let's audit these helpers to and remove unnecessary validation.