CHROMIUM: remove sysctl vm.disk_based_swap

This is unnecessary deviation w.r.t. upstream linux. The config option
CONFIG_DISK_BASED_SWAP is enough to enable/disable the swap support.

BUG=b:62107328
TEST=trybots

Change-Id: I787c706b677408f2954b74a7309aab6fbb90c58f
Reviewed-on: https://chromium-review.googlesource.com/535873
Commit-Ready: Aditya Kali <adityakali@google.com>
Tested-by: Aditya Kali <adityakali@google.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>