Make VM clear take a RelFileLocator and not fake relcache

Make VM clear take a RelFileLocator and not fake relcache

This brings it in line with visibilitymap_set(). An upcoming commit will
start reading visibility map buffers with XLogReadBuffer*() functions,
so we no longer require a relation object to pin the visibility map,
allowing us to remove this fake relcache business altogether.

While we're at it, remove a spurious const qualifier from the
RelFileLocator parameter to visibilitymap_set().

Author: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://postgr.es/m/flat/CAAKRu_bn%2Be7F4yPFBgFbnP%2BsyJRKyNK092bjD2LKvZW7O4Svag%40mail.gmail.com