led_display: remove unused DISPLAY_MARK define

DISPLAY_MARK subcommand of display_set() is not used anywhere so
we can remove it safely.

Signed-off-by: Ilya Yanok <yanok@emcraft.com>

doc/README.LED_display

@@ -14,7 +14,6 @@ This function should control the state of the LED display. Argument is
 an ORed combination of the following values:
  DISPLAY_CLEAR	-- clear the display
  DISPLAY_HOME	-- set the position to the beginning of display
- DISPLAY_MARK	-- enable mark (decimal point), if implemented
 
 int display_putc(char c);
 

include/led-display.h

@@ -29,7 +29,6 @@
 /* Display Commands */
 #define DISPLAY_CLEAR	0x1 /* Clear the display */
 #define DISPLAY_HOME	0x2 /* Set cursor at home position */
-#define DISPLAY_MARK	0x4 /* Enable the decimal point led, if implemented */
 
 void display_set(int cmd);
 int display_putc(char c);