net: remove SK_MEM_QUANTUM and SK_MEM_QUANTUM_SHIFT

Due to memcg interface, SK_MEM_QUANTUM is effectively PAGE_SIZE.

This might change in the future, but it seems better to avoid the
confusion.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>