vm_area_operations: kill ->migrate()

the only instance this method has ever grown was one in kernfs -
one that call ->migrate() of another vm_ops if it exists.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

mm/migrate.c

@@ -1536,27 +1536,6 @@ out:
 	return err;
 }
 
-/*
- * Call migration functions in the vma_ops that may prepare
- * memory in a vm for migration. migration functions may perform
- * the migration for vmas that do not have an underlying page struct.
- */
-int migrate_vmas(struct mm_struct *mm, const nodemask_t *to,
-	const nodemask_t *from, unsigned long flags)
-{
- 	struct vm_area_struct *vma;
- 	int err = 0;
-
-	for (vma = mm->mmap; vma && !err; vma = vma->vm_next) {
- 		if (vma->vm_ops && vma->vm_ops->migrate) {
- 			err = vma->vm_ops->migrate(vma, to, from, flags);
- 			if (err)
- 				break;
- 		}
- 	}
- 	return err;
-}
-
 #ifdef CONFIG_NUMA_BALANCING
 /*
  * Returns true if this is a safe migration target node for misplaced NUMA