Disconnect character movement from the mouse movements

The character could be moved forward or backward by dragging the mouse.
However, since the keyboard has already been assigned to handle
character movement, playing the game with both the mouse and keyboard
may be rather annoying. Thus, it should no longer be possible to move
using a mouse.

Author: alanjian85

src/g_game.c

@@ -403,7 +403,6 @@ void G_BuildTiccmd (ticcmd_t* cmd)
         }
     }
 
-    forward += mousey;
     if (strafe)
         side += mousex*2;
     else