arch: arm: userspace: don't subtract MPU guard from top of stack

ip register holds the stack_info.size (it is passed as argument
into z_arch_user_mode_enter(.)). We trust that the value of
stack_info.size contains the accurate size of the writable
stack buffer, above stack_info.start (as specified in kernel.h).
Therefore, we do not need to subtract any bytes for the MPU
stack guard. This allows us to clean-up one more occurrence of
CONFIG_MPU_REQUIRES_POWER_OF_TWO_ALIGNMENT in userspace.S.

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