Drop migration for ForeignReference

[wolfogre]

Fix #21086 (comment)

Related to #21721

[wxiaoguang]

I guess DropForeignReferenceTable should also be fixed together, make it ignore the error

[wolfogre]

I guess DropForeignReferenceTable should also be fixed together, make it ignore the error

Maybe it's safe to drop a nonexistent table?

[wxiaoguang]

I guess DropForeignReferenceTable should also be fixed together, make it ignore the error

Maybe it's safe to drop a nonexistent table?

Hmm, I am not sure. Some XORM behaviors are counterintuitive (I guess a better name could be DropTableIfExists()). if it's designed to do so, then no question from my side.

[zeripath]

As far as I can see xorm will tolerate dropping a non-existent table - in fact the comment preceding sess.DropTable promises this:

// DropTable drop table will drop table if exist, if drop failed, it will return error
func (session *Session) DropTable(beanOrTableName interface{}) error {
	if session.isAutoClose {
		defer session.Close()
	}

	return session.dropTable(beanOrTableName)
}

and the function dropTable is used by the code for DropTables so I think this is fine.