drivers: timer: SysTick: revert clearing pending events

Clearing the pending IRQs when resetting the timeout fixes the
forward time drifting, but the change needs more investigation
until we are sure this won't break kernel time management.
Reverting the change to get 1.14 release out.

This reverts commit 2895da02.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>