kernel/zram: remove obsolete symbol

Zsmalloc page table mappings are dead and gone [1]. Drop the respective kconfig
symbol.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=linux-5.4.y&id=69dc72f058c9b98f9b66bed184cfab7c2e9f49b0



Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>