mm: remove FGP_HEAD

This is no longer used; all callers have been converted to use folios
instead.  Somehow this manages to save 11 bytes of text.

Link: https://lkml.kernel.org/r/20221019183332.2802139-5-willy@infradead.org


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