secretmem: Remove isolate_page

The isolate_page operation is never called for filesystems, only
for device drivers which call SetPageMovable.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Hildenbrand <david@redhat.com>

mm/secretmem.c

@@ -133,11 +133,6 @@ static const struct file_operations secretmem_fops = {
 	.mmap		= secretmem_mmap,
 };
 
-static bool secretmem_isolate_page(struct page *page, isolate_mode_t mode)
-{
-	return false;
-}
-
 static int secretmem_migratepage(struct address_space *mapping,
 				 struct page *newpage, struct page *page,
 				 enum migrate_mode mode)
@@ -155,7 +150,6 @@ const struct address_space_operations secretmem_aops = {
 	.dirty_folio	= noop_dirty_folio,
 	.free_folio	= secretmem_free_folio,
 	.migratepage	= secretmem_migratepage,
-	.isolate_page	= secretmem_isolate_page,
 };
 
 static int secretmem_setattr(struct user_namespace *mnt_userns,