[mac] remove unnecessary value store (#6974)

To remove compiler warning:

  Value stored to 'caps' during its initialization is never read

src/core/mac/sub_mac.cpp

@@ -85,9 +85,10 @@ SubMac::SubMac(Instance &aInstance)
 
 otRadioCaps SubMac::GetCaps(void) const
 {
-    otRadioCaps caps = mRadioCaps;
+    otRadioCaps caps;
 
 #if OPENTHREAD_RADIO || OPENTHREAD_CONFIG_LINK_RAW_ENABLE
+    caps = mRadioCaps;
 
 #if OPENTHREAD_CONFIG_MAC_SOFTWARE_ACK_TIMEOUT_ENABLE
     caps |= OT_RADIO_CAPS_ACK_TIMEOUT;