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_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/bf038eb219bb6c7dc8a8de58de3513c035a5efba

Modified Files
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src/backend/utils/sort/logtape.c | 7 -------
1 file changed, 7 deletions(-)