procd: remove support for mounting /tmp in zram

procd: remove support for mounting /tmp in zram

The /tmp directory is mounted as tmpfs. The tmpfs filesystem is backed by
anonymous memory, which means it can be swapped out at any time, if there is
memory pressure [1]. For this reason, a zram swap device is a much better
choice than mounting /tmp on zram, since it's able to compress all anonymous
memory, and not just the memory assigned to /tmp. We already have the zram-swap
package for this specific purpose, which means procd's tmp-on-zram is both
redundant and more limited.

A follow-up patch will remove support for mounting /tmp in zram from procd
itself.

[1] https://www.kernel.org/doc/Documentation/filesystems/tmpfs.txt

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