interval.sql

Commits a7145f6b et al. added a test to verify integer overflow
detection in interval_mul.  That only applies with integer timestamps,
of course, so it's problematic in pre-v10 branches where we supported
float timestamps.  The test was only marginally worth the trouble to
begin with, so just remove it in those branches.  Per buildfarm.

Discussion: https://postgr.es/m/6437.1573319193@sss.pgh.pa.us