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/Models/Asset.php

@@ -3,6 +3,7 @@ namespace App\Models;
 
 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 @@ class Asset extends Depreciable
         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;