Remove faulty Assert in partitioned INSERT...ON CONFLICT DO UPDATE.

Remove faulty Assert in partitioned INSERT...ON CONFLICT DO UPDATE.

Commit f16241bef mistakenly supposed that INSERT...ON CONFLICT DO
UPDATE rejects partitioned target tables.  (This may have been
accurate when the patch was written, but it was already obsolete
when committed.)  Hence, there's an assertion that we can't see
ItemPointerIndicatesMovedPartitions() in that path, but the assertion
is triggerable.

Some other places throw error if they see a moved-across-partitions
tuple, but there seems no need for that here, because if we just retry
then we get the same behavior as in the update-within-partition case,
as demonstrated by the new isolation test.  So fix by deleting the
faulty Assert.  (The fact that this is the fix doubtless explains
why we've heard no field complaints: the behavior of a non-assert
build is fine.)

The TM_Deleted case contains a cargo-culted copy of the same Assert,
which I also deleted to avoid confusion, although I believe that one
is actually not triggerable.

Per our code coverage report, neither the TM_Updated nor the
TM_Deleted case were reached at all by existing tests, so this
patch adds tests for both.

Reported-by: Dmitry Koval <d.koval@postgrespro.ru>
Author: Joseph Koshakow <koshy44@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/f5fffe4b-11b2-4557-a864-3587ff9b4c36@postgrespro.ru
Backpatch-through: 14