Deleted unused variable

Deleted the var GRAVITY because it is unused. The gravity is used at line 237, but it's gotten from the Physics2DDirectBodyState parameter.

demos/2d/platformer/player.gd

@@ -33,7 +33,6 @@ var shooting=false
 var WALK_ACCEL = 800.0
 var WALK_DEACCEL= 800.0
 var WALK_MAX_VELOCITY= 200.0
-var GRAVITY = 700.0
 var AIR_ACCEL = 200.0
 var AIR_DEACCEL= 200.0
 var JUMP_VELOCITY=460