pgsql: Remove faulty nbtree page deletion assertion.

Remove faulty nbtree page deletion assertion.

_bt_pagedel only builds a search stack when it needs one to locate the
downlink of the page that it's about to mark half-dead (and when it
doesn't already have one from an earlier sibling deletion that took
place during the same _bt_pagedel call). _bt_pagedel won't build a
search stack for the page its caller passes whenever it happens to be
one that an earlier interrupted VACUUM left half-dead (since there is no
downlink to remove). We nevertheless asserted that page deletion could
only build a search stack for that initial caller-passed page.

To fix, remove the assertion, which spuriously failed when a half-dead
leaf page was passed by the _bt_pagedel caller and its right sibling
page was empty/had to be considered for deletion during the same call
(necessitating building a search stack to locate the right sibling's
downlink).

Oversight in commit cd8c73a38, which refactored nbtree page deletion's
INCOMPLETE_SPLIT check.

Author: Peter Geoghegan <pg(at)bowt(dot)ie>
Reported-by: Iliia Khaprov <i(dot)khaprov(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAFO+O_p5So4MXwQ1VEcL9+9vGgODu11TTWcYq9RDbxnira8o2w@mail.gmail.com
Backpatch-through: 14

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3b7c13ec54b157dcda292d82c320510904e99531

Modified Files
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src/backend/access/nbtree/nbtpage.c | 11 +++++------
1 file changed, 5 insertions(+), 6 deletions(-)