Remove redundant and ineffective test for btree insertion fast path.

indexing.sql's test for this feature was added along with the
feature in commit 2b2727343.  However, shortly later that test was
rendered ineffective by commit 074251db6, which limited when the
optimization would be applied, so that the test didn't test it.
Since then, commit dd299df81 added new tests (in btree_index.sql)
that actually do test the feature.  Code coverage comparisons
confirm that this test sequence adds no meaningful coverage, and
it's rather expensive, accounting for nearly half of the runtime
of indexing.sql according to my measurements.  So let's remove it.

Per advice from Peter Geoghegan.

Discussion: https://postgr.es/m/735.1554935715@sss.pgh.pa.us

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