luci-mod-system/zram: remove zram_comp_streams

Zram uses per-CPU compression streams [1]. This is a no-op.

[1] https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=116191eddf9d8ddba61de788824dea442b265936

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