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

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

src/test/regress/expected/interval.out

@@ -232,9 +232,6 @@ INSERT INTO INTERVAL_TBL_OF (f1) VALUES ('-2147483648 years');
 ERROR:  interval out of range
 LINE 1: INSERT INTO INTERVAL_TBL_OF (f1) VALUES ('-2147483648 years'...
                                                  ^
--- Test edge-case overflow detection in interval multiplication
-select extract(epoch from '256 microseconds'::interval * (2^55)::float8);
-ERROR:  interval out of range
 SELECT r1.*, r2.*
    FROM INTERVAL_TBL_OF r1, INTERVAL_TBL_OF r2
    WHERE r1.f1 > r2.f1

src/test/regress/sql/interval.sql

@@ -73,9 +73,6 @@ INSERT INTO INTERVAL_TBL_OF (f1) VALUES ('-2147483649 days');
 INSERT INTO INTERVAL_TBL_OF (f1) VALUES ('2147483647 years');
 INSERT INTO INTERVAL_TBL_OF (f1) VALUES ('-2147483648 years');
 
--- Test edge-case overflow detection in interval multiplication
-select extract(epoch from '256 microseconds'::interval * (2^55)::float8);
-
 SELECT r1.*, r2.*
    FROM INTERVAL_TBL_OF r1, INTERVAL_TBL_OF r2
    WHERE r1.f1 > r2.f1