How do i get rid of my Crosshair when i aim?

You would make a boolean which gets true or false at certain conditions. Now nothing from the Input-class should live inside the OnGUI function, so we have to go around that:

 private var isAiming : boolean = false;
 
 function OnGUI () {
     if (isAiming) {
         //Hide the crosshair
     } else {
         //Show the crosshair
     }
 }
 
 function Update () {
     if (Input.GetButtonDown("Fire2")) isAiming = true;
     if (Input.GetButtonUp("Fire2")) isAiming = false;
 }

If your crosshair isn't a GUI component, rather a GameObject living in 3d space, same method would apply but you can cut the intermediary boolean:

 var crosshairRenderer : Renderer;
 
 function Update () {
     if (Input.GetButtonDown("Fire2")) crosshairRenderer.enabled = false;
     if (Input.GetButtonUp("Fire2")) crosshairRenderer.enabled = true;
 }

Lastly you could also work with the alpha channels of your crosshair (3d space example):

 var crosshairRenderer : Renderer;
 var crosshairShowHideSpeed = 5.0;
 
 function Update () {
     if (Input.GetButtonDown("Fire2")) HideCrosshair();
     if (Input.GetButtonUp("Fire2")) ShowCrosshair();
 }
 
 function HideCrosshair () {
     while (Input.GetButton("Fire2") && crosshairRenderer.material.color.a>.0) {
         crosshairRenderer.material.color.a -= crosshairShowHideSpeed*Time.deltaTime;
         yield;
     }
 }
 
 function ShowCrosshair () {
     var stop : boolean = false;
     while (!stop && crosshairRenderer.material.color.a<1.0) {
         crosshairRenderer.material.color.a += crosshairShowHideSpeed*Time.deltaTime;
         if (Input.GetButton("Fire2")) stop = true;
         yield;
     }
 }