mm/unicore32: fix stale comment about VALID_PAGE()

VALID_PAGE() has been removed from kernel long time ago,
so fix the comment.
Signed-off-by: NJiang Liu <jiang.liu@huawei.com>
Acked-by: NGuan Xuetao <gxt@mprc.pku.edu.cn>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/unicore32/include/asm/memory.h

@@ -98,12 +98,6 @@
 /*
  * Conversion between a struct page and a physical address.
  *
- * Note: when converting an unknown physical address to a
- * struct page, the resulting pointer must be validated
- * using VALID_PAGE().  It must return an invalid struct page
- * for any physical address not corresponding to a system
- * RAM address.
- *
  *  page_to_pfn(page)	convert a struct page * to a PFN number
  *  pfn_to_page(pfn)	convert a _valid_ PFN number to struct page *
  *