pgsql: Silence Coverity warning about unused visibilitymap_clear() resu

Silence Coverity warning about unused visibilitymap_clear() result

Commit c0d9864f5ce made all but one caller of visibilitymap_clear() check
its return value, causing Coverity to flag the remaining unchecked call
in heap_page_fix_vm_corruption().

This VM clear is not WAL-logged, so the caller doesn't need the return
value of visibilitymap_clear(). Add an explicit void cast and comment to
document that the return value is intentionally ignored.

Backpatch to 19 when the number of callers discarding the return value
dropped low enough to trigger Coverity's warning.

Discussion: https://postgr.es/m/1065814.1784514869%40sss.pgh.pa.us
Reported-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Backpatch-through: 19

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/2f768dda531c94c071972a68d77163fba35684a6

Modified Files
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src/backend/access/heap/pruneheap.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)