target/arm: remove unused EXCP_SEMIHOST leg

All semihosting exceptions are dealt with earlier in the common code
so we should never get here.

Backports commit b906acbb3aceed5b1eca30d9d365d5bd7431400b from qemu

qemu/target/arm/helper.c

@@ -8331,13 +8331,6 @@ static void arm_cpu_do_interrupt_aarch64_(CPUState *cs)
     case EXCP_VFIQ:
         addr += 0x100;
         break;
-    case EXCP_SEMIHOST:
-        /* UNICORN: Commented out
-        qemu_log_mask(CPU_LOG_INT,
-                      "...handling as semihosting call 0x%" PRIx64 "\n",
-                      env->xregs[0]);
-        env->xregs[0] = do_arm_semihosting(env);*/
-        return;
     default:
         cpu_abort(cs, "Unhandled exception 0x%x\n", cs->exception_index);
     }