highmem: Remove unused functions

Nothing uses totalhigh_pages_dec() and totalhigh_pages_set().

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: https://lore.kernel.org/r/20201103095856.732891880@linutronix.de