arch: arm: userspace: no PSPLIM clearing in z_arm_do_syscall() enter

In this commit we remove the PSPLIM clearing when entering
z_arm_do_syscall(), since we want PSPLIM to keep guarding
the user thread stack, until the thread has switched to its
privileged stack, for executing the system call.

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