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 2895da0.

Signed-off-by: Ioannis Glaropoulos <[email protected]>

Author: ioannisg

drivers/timer/cortex_m_systick.c

@@ -147,22 +147,6 @@ void z_clock_set_timeout(s32_t ticks, bool idle)
 	SysTick->LOAD = last_load - 1;
 	SysTick->VAL = 0; /* resets timer to last_load */
 
-	/*
-	 * In the unlucky scenario of a SysTick event (wrap) occurring
-	 * while we re-program the last_load value, the SysTick ISR
-	 * will run immediately after we unlock interrupts. In that
-	 * case the timeout we have just configured will expire
-	 * instantaneously, leading to operations being executed
-	 * much earlier than expected. Avoid this by clearing possibly
-	 * pending SysTick exceptions (writing 1 to ICSR.PENDSTCLR).
-	 *
-	 * Side effect: any operations that were scheduled to execute
-	 * upon the expiration of the timeout we may be canceling (by
-	 * writing PENDSTCLR to 1) will be postponed, until the new
-	 * timeout (i.e. the one we are programming now) expires.
-	 */
-	SCB->ICSR |= SCB_ICSR_PENDSTCLR_Msk;
-
 	k_spin_unlock(&lock, key);
 #endif
 }