arch: arm: aarch32: no PSLIM clearing in z_arm_userspace_enter()

Modifying the PSP via an MSR instruction is not subject to
stack limit checking so we can remove the relevant code
block in the begining of z_arm_userspace_enter(), which clears
PSPLIM. We add a comment when setting the PSP to the privilege
stack to stress that clearing the PSPLIM is not required and it
is always a safe operation.

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