Remove zram_swap file after zram writeback setup

/data/per_boot/zram_swap is created for zram writeback but it doesn't
need to exist for zram writeback to work once the loop device is
successfully created. This will also make sure only zram driver has
access to the writeback file and improve security.

Bug: 390004071
Test: manual, verify /data/per_boot is empty after zram setup from
swapon_all

Change-Id: Ifa4755baad090cf34d26e7d572431b6345412043