Remove duplicate game.controllerSensitivity proxy

It wasn't a direct duplicate of key.sensitivity, but it was still
basically the same thing. Although to be fair, at least the case-switch
conversion didn't get duplicated everywhere unlike game.slowdown.

So now key.sensitivity functions the same as game.controllerSensitivity,
and it only gets converted to its actual value whenever a joystick input
happens in key.Poll(), unlike previously where it got converted every
single frame on the title screen (there was even a comment that said
"TODO bit wasteful doing this every poll").

desktop_version/src/Game.h

@@ -366,8 +366,6 @@ public:
     bool savemystats;
 
 
-    int controllerSensitivity;
-
     bool quickrestartkludge;
 
     //Custom stuff