The counter dict_table_t::n_foreign_key_checks_running
was a work-around for missing meta-data locking (MDL)
on the SQL layer.

ER_TABLE_IN_FK_CHECK: Replaced with ER_UNUSED_26.

HA_ERR_TABLE_IN_FK_CHECK: Remove.

dict_table_t: Remove inc_fk_checks(), dec_fk_checks(),
n_foreign_key_checks_running.

row_ins_check_foreign_constraints(),
row_upd_check_references_constraints(): Do not acquire
a shared dict_sys latch nor touch the reference counters.

row_drop_table_for_mysql(): Make the checks stricter.
We will still employ row_mysql_drop_list for
TRUNCATE TABLE and for dropping internal tables related
to FULLTEXT INDEX. So, this will not fix MDEV-21283 yet.

MDEV-21602 CREATE TABLE…PRIMARY KEY…SELECT workaround
causes DROP TABLE to ignore locks

The error handling of CREATE…SELECT would invoke
handler::delete_table() while still holding locks
on the table, due to not having invoked handlerton::rollback
first. InnoDB used to work around this as well.
In MDEV-742 this was worked around further by breaking
MDL, causing MDEV-22733.