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

contrib/amcheck/verify_heapam.c

@@ -853,8 +853,6 @@ verify_heapam(PG_FUNCTION_ARGS)
 			break;
 	}
 
-	/* Ensure that the stream is completely read */
-	Assert(read_stream_next_buffer(stream, NULL) == InvalidBuffer);
 	read_stream_end(stream);
 
 	if (vmbuffer != InvalidBuffer)

src/backend/access/gist/gistvacuum.c

@@ -254,8 +254,6 @@ gistvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
 			gistvacuumpage(&vstate, buf);
 		}
 
-		Assert(read_stream_next_buffer(stream, NULL) == InvalidBuffer);
-
 		/*
 		 * We have to reset the read stream to use it again. After returning
 		 * InvalidBuffer, the read stream API won't invoke our callback again

src/backend/access/nbtree/nbtree.c

@@ -1117,8 +1117,6 @@ btvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
 											 current_block);
 		}
 
-		Assert(read_stream_next_buffer(stream, NULL) == InvalidBuffer);
-
 		/*
 		 * We have to reset the read stream to use it again. After returning
 		 * InvalidBuffer, the read stream API won't invoke our callback again

src/backend/access/spgist/spgvacuum.c

@@ -879,8 +879,6 @@ spgvacuumscan(spgBulkDeleteState *bds)
 				spgprocesspending(bds);
 		}
 
-		Assert(read_stream_next_buffer(stream, NULL) == InvalidBuffer);
-
 		/*
 		 * We have to reset the read stream to use it again. After returning
 		 * InvalidBuffer, the read stream API won't invoke our callback again