Change farm_timer to uint8_t

Remove extern declaration of farm_timer and farm_status as they
are only used in one file.

Saves 26 bytes of flash

Firmware/ultralcd.cpp

@@ -80,10 +80,9 @@ static uint8_t lcd_commands_step = 0;
 CustomMsg custom_message_type = CustomMsg::Status;
 uint8_t custom_message_state = 0;
 
-
 bool isPrintPaused = false;
 uint8_t farm_mode = 0;
-int farm_timer = 8;
+uint8_t farm_timer = 8;
 uint8_t farm_status = 0;
 bool printer_connected = true;
 

Firmware/ultralcd.h

@@ -124,8 +124,6 @@ extern CustomMsg custom_message_type;
 extern uint8_t custom_message_state;
 
 extern uint8_t farm_mode;
-extern int farm_timer;
-extern uint8_t farm_status;
 
 extern bool UserECoolEnabled();
 extern bool FarmOrUserECool();