Remove unneeded nbtree array preprocessing assert.

Certain cases involving the use of cursors had assertion failures within
_bt_preprocess_keys's recently added no-op return path.  The assertion
in question made the faulty assumption that a second or third call to
_bt_preprocess_keys (within the same btrescan) could only happen when
another scheduled primitive index scan was just about to begin.

It would be possible to address the problem by only allowing scans that
have array keys to take the new no-op path, forcing affected cases to
perform redundant preprocessing work.  It seems simpler to just remove
the assertion, and reframe the no-op path as a more general mechanism.
Take this simpler approach.

The important underlying principle is that we only need to perform
preprocessing once per btrescan (at most).  This is expected regardless
of whether or not the scan happens to have array keys.

Oversight in commit 1b134ca5, which enhanced nbtree ScalarArrayOp
execution.

Reported-By: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/ef0f7c8b-a6fa-362e-6fd6-054950f947ca@gmail.com

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