mm: remove lookup_swap_cache()

All callers have now been converted to swap_cache_get_folio(), so we can
remove this wrapper.

Link: https://lkml.kernel.org/r/20220902194653.1739778-39-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>