Remove table AM callback scan_bitmap_next_block

After pushing the bitmap iterator into table-AM specific code (as part
of making bitmap heap scan use the read stream API),
scan_bitmap_next_block() no longer returns the current block number.
Since scan_bitmap_next_block() isn't returning any relevant information
to bitmap table scan code, it makes more sense to get rid of it.

Now, bitmap table scan code only calls table_scan_bitmap_next_tuple(),
and the heap AM implementation of scan_bitmap_next_block() is a local
helper in heapam_handler.c.

Reviewed-by: Tomas Vondra <[email protected]>
Discussion: https://postgr.es/m/d4bb26c9-fe07-439e-ac53-c0e244387e01%40vondra.me

ci-os-only:

Author: melanieplageman