Am I able to remove unneeded field/migration tables from the database manually? I have been working on my site for three months now and I have noticed that the number of empty tables in my database has grown considerably.  Upon closer inspection, I saw that these tables were use by fields belonging to content types that have been removed from the site.  Apparently Drupal does a bad job of removing field tables when you simply delete a content type.  Is it OK for me to delete these tables manually, or will doing this break code (I have been seeing plenty of mysterious, field related errors lately.)  If so, is there anything else besides the field table that I can remove? 

The same question is also asked about the migration tables.  Can I get rid of them if they don't go away?
Yes, use

drush migrate-deregister --orphans


This should get rid of all migration tracking tables that are not longer being referred to.