Remove abstime, reltime, tinterval tables from old regression databases.

In the back branches, drop these tables after the regression tests are
done with them.  This fixes failures of cross-branch pg_upgrade testing
caused by these types having been removed in v12.  We do lose the ability
to test dump/restore behavior with these types in the back branches, but
the actual loss of code coverage seems to be nil given that there's nothing
very special about these types.

Discussion: https://postgr.es/m/20181009192237.34wjp3nmw7oynmmr@alap3.anarazel.de