[libcpu/arm] remove useless cpsr updating in rt_hw_context_switch_to

This piece of code will enable the interrupt early before switching to
the first thread. Although it is harmless, but not prefect.

libcpu/arm/realview-a8-vmm/context_gcc.S

@@ -57,9 +57,6 @@ rt_hw_context_switch_to:
     ldmfd sp!, {r4}         @ pop new task spsr
     msr spsr_cxsf, r4
 
-    bic r4, r4, #0x20       @ must be ARM mode
-    msr cpsr_cxsf, r4
-
     ldmfd sp!, {r0-r12, lr, pc}^   @ pop new task r0-r12, lr & pc
 
 .section .bss.share.isr