Creation of tables by the three names mysql.user, mysql.host, mysql.db
was being blocked in the function row_mysql_is_system_table().

Since commit 4abb8216 (MDEV-17658),
mysql.user is a view, not a table. Since commit
ead9a34a (MDEV-15851), mysql.host
is not being created at all.

Let us remove the special handling of table names in InnoDB,
and allow mysql.db to be created in InnoDB. The special handling
was originally added in commit e84ef2b7
without any explanation.

Reviewed by: Sergei Golubchik