Remove migration of shortcuts from menu_link migration

To other committers, I'll do this one in a few days.

Updating title and credit

This is changing the behavior of a source plugin such that shortcut links are not returned. In this case, that is not an problem because the saving a shortcut with the menu_link destination causes an error. This is cleaning up that error. I originally suggested that the assertion test for the text of the error message. Instead, the patch checks that there are zero error messages. That may be slightly better in that it increases the likelihood of failing due to other introduced errors.

Committed 679dff2 and pushed to 10.1.x. Thanks!

And cherry picked to 10.0.x and 9.5.x

Automatically closed - issue fixed for 2 weeks with no activity.