arch: arm: userspace: remove mode setting in swap()

The arch.mode status flag is always inline with the actual
thread execution privilege leve. Therefore, we do not need
to be saving-off the thread's mode every time we intend to
context switch-out the thread.

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