pgsql: Remove test case that doesn't work with --disable-integer-timest

Remove test case that doesn't work with --disable-integer-timestamps.

Commits a7145f6bc 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

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/30f6998ffc7df1f5d27cf40f20ac550ab4be5bfa

Modified Files
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src/test/regress/expected/interval.out | 3 ---
src/test/regress/sql/interval.sql | 3 ---
2 files changed, 6 deletions(-)