pgsql: Remove incorrect Assert introduced in c8aeaf3ab.

Remove incorrect Assert introduced in c8aeaf3ab.

Already removed incidentally in version 15 (c4649cce3), so this commit
is only applied to versions 13 and 14.

The comment above is misleading in all versions 13 and later, so that
will be fixed in a separate commit.

Discussion: https://postgr.es/m/cfd84cb8-12fe-433a-a4bb-f460a4515f9c.zhaotinghai.zth%40alibaba-inc.com
Reported-by: Tinghai Zhao
Backpatch-through: 13

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a1a51dc4fab7efdcf3c3a50d9dd9f0a4a420f4f7

Modified Files
--------------
src/backend/utils/sort/logtape.c | 8 --------
1 file changed, 8 deletions(-)