Fix compiler errors

src/TinyGsmClientA6.h

@@ -289,7 +289,8 @@ public:
 
   bool radioOff() TINY_GSM_ATTR_NOT_IMPLEMENTED;
 
-  bool sleepEnable(bool enable = true) TINY_GSM_ATTR_NOT_IMPLEMENTED;
+  bool sleepEnable(bool enable = true) { return false; }
+  // TINY_GSM_ATTR_NOT_IMPLEMENTED;
 
   /*
    * SIM card functions

src/TinyGsmClientESP8266.h

@@ -302,7 +302,8 @@ public:
 
   bool radioOff() TINY_GSM_ATTR_NOT_IMPLEMENTED;
 
-  bool sleepEnable(bool enable = true) TINY_GSM_ATTR_NOT_IMPLEMENTED;
+  bool sleepEnable(bool enable = true) { return false; }
+  // TINY_GSM_ATTR_NOT_IMPLEMENTED;
 
   /*
    * SIM card functions

src/TinyGsmClientUBLOX.h

@@ -320,7 +320,8 @@ public:
     return true;
   }
 
-  bool sleepEnable(bool enable = true) TINY_GSM_ATTR_NOT_IMPLEMENTED;
+  bool sleepEnable(bool enable = true) { return false; }
+  // TINY_GSM_ATTR_NOT_IMPLEMENTED;
 
   /*
    * SIM card functions

src/TinyGsmCommon.h

@@ -250,8 +250,6 @@ public:
    */
 
   virtual bool restart() = 0;
-  virtual bool poweroff() { return false; }
-  virtual bool radioOff() { return false; }
   virtual bool sleepEnable(bool enable = true) { return false; }
 
   /*