mm: remove struct pagevec

All users are now converted to use the folio_batch so we can get rid of
this data structure.

Link: https://lkml.kernel.org/r/20230621164557.3510324-11-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>