btr_node_ptr_max_size(): Let us remove the debug assertion that was
added in MDEV-14637. The assertion assumed that no additional
indexes exist in mysql.innodb_index_stats or mysql.innodb_table_stats.
The code path is working around an incorrect definition of a table,
interpreting VARCHAR(64) as the more correct VARCHAR(199).

No test case will be added, because MDEV-24579 proves that executing
DDL on the statistics tables involves a race condition. The test
case included the following:

	ALTER TABLE mysql.innodb_index_stats ADD KEY (stat_name);
	CREATE TABLE t (a INT) ENGINE=InnoDB STATS_PERSISTENT=1;