mm/mmu_gather: Remove encoded_page infrastructure

commit 70fb4fdf ("mm: introduce 'encoded' page pointers with
embedded extra bits") and commit 7cc8f9c7 ("mm: mmu_gather: prepare
to gather encoded page pointers with flags") converted mmu_gather for
dealing with encoded_page, where the bottom 2 bits could encode extra
flags. Only 1 bit was ever used; to flag whether the page should
participate in a delayed rmap removal.

Now that the mmu_gather batched rmap removal mechanism has been
generalized, all pages participate and therefore the flag is unused. So
let's remove encoded_page to simplify the code. It also gets in the way
of further optimization which will be done in a follow up patch.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>