soc: qcom: Remove watchdog slack accounting

Refactor watchdog core to remove dead code. Specifically,
the slack variables from the watchdog data structure and
the pet function that uses them. Currently these variables
are not used and can be refactored to simply reset the
watchdog counter directly.

Change-Id: Ie184e2826ed790288a94dfebf578ec99b53831e8
Signed-off-by: Branden Bonaby <bbonaby@codeaurora.org>