pgsql: Remove misleading read stream asserts in a few users

Remove misleading read stream asserts in a few users

Several read stream users asserted that the read stream was exhausted
after looping on that very condition. It was pointed out in an a
review of an as-of-yet uncommitted read stream user [1] that this was
confusing and could lead the reader to think there was a possibility of
some kind of race condition. Remove these asserts.

[1] https://postgr.es/m/F9ACE8D0-B807-4A17-B6BD-87EF0717983D%40yesql.se

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/54a3615f15b9b2e1433bc47cfb150317885d7c1f

Modified Files
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contrib/amcheck/verify_heapam.c | 2 --
src/backend/access/gist/gistvacuum.c | 2 --
src/backend/access/nbtree/nbtree.c | 2 --
src/backend/access/spgist/spgvacuum.c | 2 --
4 files changed, 8 deletions(-)