MDEV-17831 Assertion `supports_instant()' failed in dict_table_t::prepare_instant upon ADD COLUMN on table with KEY_BLOCK_SIZE

dict_table_t::prepare_instant(): Remove too strict assertions that
could fail if the MariaDB and InnoDB data dictionaries are out of sync.

That the dictionaries got out of sync during TRUNCATE TABLE
is a separate bug was fixed in 10.2 by
commit 971e1d86.