mm: remove page_evictable()

Patch series "Remove leftover mlock/munlock page wrappers".

We no longer need the various mlock page functions as all callers have
folios.


This patch (of 4):

This function now has no users.  Also update the unevictable-lru
documentation to discuss folios instead of pages (mostly).

[akpm@linux-foundation.org: fix Documentation/mm/unevictable-lru.rst underlining]
  Link: https://lkml.kernel.org/r/20230117145106.585b277b@canb.auug.org.au
Link: https://lkml.kernel.org/r/20230116192827.2146732-1-willy@infradead.org
Link: https://lkml.kernel.org/r/20230116192827.2146732-2-willy@infradead.org


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