mm/page_alloc.c: drop dead destroy_compound_page()

The only caller is __free_one_page(). By the time we should have
page->flags to be cleared already:

 - for 0-order pages though PCP list:
	free_hot_cold_page()
		free_pages_prepare()
			free_pages_check()
				page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
		<put the page to PCP list>

	free_pcppages_bulk()
		page = <withdraw pages from PCP list>
		__free_one_page(page)

 - for non-0-order pages:
	__free_pages_ok()
		free_pages_prepare()
			free_pages_check()
				page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
		free_one_page()
			__free_one_page()

So there's no way PageCompound() will return true in __free_one_page().
Let's remove dead destroy_compound_page() and put assert for page->flags
there instead.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>