arch-arm: Remove unimplemented miscreg handling in MSR imm

The checkFaultAccessAArch64SysReg function is now in charge
of returning the Undefined Instruction fault for unimplemented
registers

Change-Id: I75c00c6fbce33d1729a4923eae585260a8461db3
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/61971
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/arm/isa/insts/data64.isa

@@ -534,12 +534,6 @@ let {{
             MiscRegIndex misc_index = (MiscRegIndex) xc->tcBase()->
                 flattenRegId(miscRegClass[pre_flat]).index();
 
-            if (!lookUpMiscReg[misc_index].info[MISCREG_IMPLEMENTED]) {
-                    return std::make_shared<UndefinedInstruction>(
-                                    machInst, false,
-                                    mnemonic);
-            }
-
             if (fault = checkFaultAccessAArch64SysReg(misc_index,
                 Cpsr, xc->tcBase(), *this); fault != NoFault) {
                     return fault;