During a table-rebuilding operation, the function table_name_parse()
can encounter a table name that starts with #sql. Here is an example
of a failure:

CURRENT_TEST: gcol.innodb_virtual_basic
mysqltest: At line 1204: query 'alter table t drop column d ' failed:
2013: Lost connection to MySQL server during query

Let us just remove these bogus debug assertions.

If the final renaming phase during ALTER TABLE never fails, it
should not do any harm to skip the purge. If it does fail, then
we might end up 'leaking' some delete-marked records in the
indexes on virtual columns of the original table, and these
garbage records would keep consuming space until the indexes are
dropped or the table is successfully rebuilt.