Revert "arch: arm: cortex_m: Only trigger context switch if thread is preemptible"

This reverts commit 42036cdb.

Architecture specific code should not do preemption checking before context
switch. This is already handled by the scheduler, so duplicating it would
be redundant and error prone. These checks used to be necessary, but the
scheduler has been rewritten since then and the checks were removed in
3a0cb2d3 (kernel: Remove legacy preemption checking, 2018-05-23).

The check this reverts was also incorrect, as it didn't take scheduler
locking nor meta-IRQs into account.

Fixes #80574

Signed-off-by: Kalle Kietäväinen <kalle.kietavainen@silabs.com>