Revert "Added two new tables, exercise_user_pairs and workout_user pairs to keep track of who "owns" which exercises."

This schema is becoming a bit too convoluted for our purposes.  I'm just going to add user_id ints to workout_table and exercise_table.

This reverts commit a32482e292bba9a5951853a16f080a358c8b7f93.
Merge branch 'master' of https://github.com/kryzodoze/340-project