[low-power] fix CSL_RECEIVER build option (#5428)

This commit removes the config setting
OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE automatically when it's 1.2
and MTD. The reason is that it's in core config and it's also used in
platform (radio) which cannot access core config.

If CSL_RECEIVER is not set in options in cmake and it's 1.2, then in
core config, OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE would be turned
on if it's MTD. While in platform, it's not turned on. And then there
would be a link issue.

src/core/config/mac.h

@@ -318,12 +318,10 @@
  * @def OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
  *
  * This setting configures the CSL receiver feature in Thread 1.2.
- * This is compulsory for 1.2 MTD, optional for 1.2 FTD.
  *
  */
 #ifndef OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
-#define OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE \
-    (OPENTHREAD_CONFIG_THREAD_VERSION >= OT_THREAD_VERSION_1_2) && OPENTHREAD_MTD
+#define OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE 0
 #endif
 
 #if OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE

src/core/radio/radio.hpp

@@ -413,6 +413,7 @@ public:
      */
     otError Receive(uint8_t aChannel) { return otPlatRadioReceive(GetInstance(), aChannel); }
 
+#if OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
     /**
      * This method updates the CSL sample time in radio.
      *
@@ -436,6 +437,7 @@ public:
     {
         return otPlatRadioEnableCsl(GetInstance(), aCslPeriod, aExtAddr);
     }
+#endif // OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
 
     /**
      * This method gets the radio transmit frame buffer.