InnoDB used to support at most one CREATE TABLE or DROP TABLE
per transaction. This caused complications for DDL operations on
partitioned tables (where each partition is treated as a separate
table by InnoDB) and FULLTEXT INDEX (where each index is maintained
in a number of internal InnoDB tables).

dict_drop_index_tree(): Extend the MDEV-24589 logic and treat
the purge or rollback of SYS_INDEXES records of clustered indexes
specially: by dropping the tablespace if it exists. This is the only
form of recovery that we will need.

trx_undo_ddl_type: Document the DDL undo log record types better.

trx_t::dict_operation: Change the type to bool.

trx_t::ddl: Remove.

trx_t::table_id, trx_undo_t::table_id: Remove.

dict_build_table_def_step(): Remove trx_t::table_id logging.

dict_table_close_and_drop(), row_merge_drop_table(): Remove.

row_merge_lock_table(): Merged to the only callers, which can
call lock_table_for_trx() directly.

fts_aux_table_t, fts_aux_id, fts_space_set_t: Remove.

fts_drop_orphaned_tables(): Remove.

row_merge_rename_index_to_drop(): Remove. Thanks to MDEV-24589,
we can simply delete the to-be-dropped indexes from SYS_INDEXES,
while still being able to roll back the operation.

ha_innobase_inplace_ctx: Make a few data members const.
Preallocate trx.

prepare_inplace_alter_table_dict(): Simplify the logic. Let the
normal rollback take care of some cleanup.

row_undo_ins_remove_clust_rec(): Simplify the parsing of SYS_COLUMNS.

trx_rollback_active(): Remove the special DROP TABLE logic.

trx_undo_mem_create_at_db_start(), trx_undo_reuse_cached():
Always write TRX_UNDO_TABLE_ID as 0.