Revert asset-checkout-different validation.

This was causing issues when trying to check an item out to a user or a
location because of the way laravel handles validation.

Instead, rely on the exception check we had in the controller.  I moved
this exception up to the model checkout method so that it would work
for anywhere that that method was called, even if it avoided the
controller.

app/Http/Controllers/Assets/AssetCheckoutController.php

@@ -62,9 +62,7 @@ public function store(AssetCheckoutRequest $request, $assetId)
             $admin = Auth::user();
 
             $target = $this->determineCheckoutTarget($asset);
-            if ($asset->is($target)) {
-                throw new CheckoutNotAllowed('You cannot check an asset out to itself.');
-            }
+
             $asset = $this->updateAssetLocation($asset, $target);
 
             $checkout_at = date("Y-m-d H:i:s");

app/Http/Requests/AssetCheckoutRequest.php

@@ -23,7 +23,7 @@ public function rules()
     {
         $rules = [
             "assigned_user"         => 'required_without_all:assigned_asset,assigned_location',
-            "assigned_asset"        => 'required_without_all:assigned_user,assigned_location|different:'.$this->id,
+            "assigned_asset"        => 'required_without_all:assigned_user,assigned_location',
             "assigned_location"     => 'required_without_all:assigned_user,assigned_asset',
             "checkout_to_type"      => 'required|in:asset,location,user'
         ];

app/Models/Asset.php

@@ -3,6 +3,7 @@
 
 use App\Events\AssetCheckedOut;
 use App\Events\CheckoutableCheckedOut;
+use App\Exceptions\CheckoutNotAllowed;
 use App\Http\Traits\UniqueSerialTrait;
 use App\Http\Traits\UniqueUndeletedTrait;
 use App\Models\Traits\Acceptable;
@@ -271,6 +272,9 @@ public function checkOut($target, $admin = null, $checkout_at = null, $expected_
         if (!$target) {
             return false;
         }
+        if ($this->is($target)) {
+            throw new CheckoutNotAllowed('You cannot check an asset out to itself.');
+        }
 
         if ($expected_checkin) {
             $this->expected_checkin = $expected_checkin;