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

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