the implementation would be challenging with ROW_FORMAT=COMPACT.

Remove the table types DICT_TABLE_CLUSTER_MEMBER and DICT_TABLE_CLUSTER and
all related tests and functions.

dict_table_t: Remove mix_id, mix_len, mix_id_len, mix_id_buf, and
cluster_name.

plan_t: Remove mixed_index.

dict_create_sys_tables_tuple(): Set MIX_ID=0, MIX_LEN=0 and CLUSTER_NAME=NULL
when inserting into SYS_TABLES.

dict_tree_check_search_tuple(): Enclose in #ifdef UNIV_DEBUG.