BLE - Security Manager PAL: Add a function to remove the peer csrk.

If the the upper layer has registered a peer csrk on the pal security manager then it must remove it once the connection is closed.

This API allows the upper layer to remove the peer csrk registered earlier.

features/FEATURE_BLE/ble/pal/PalSecurityManager.h

@@ -883,6 +883,8 @@ public:
         sign_count_t sign_counter
     ) = 0;
 
+    virtual ble_error_t remove_peer_csrk(connection_handle_t connection) = 0;
+
     ////////////////////////////////////////////////////////////////////////////
     // Authentication
     //