add the possibility to remove a category from a thirdparty

I don't know if there is a good reason for not proposing to unlink a thirdparty from a category but if it is not the case here is a piece of code allowing to remove a category that works for me and seems general enough to be useful to other people.
The new function is inspired from the addCategory function.

PS : This is the first time i'm trying to contribute on github so don't hesitate to tell me if I did something wrong.

htdocs/societe/class/api_thirdparties.class.php

@@ -552,7 +552,45 @@ class Thirdparties extends DolibarrApi
       return $this->company;
     }
 
+    /**
+     * Delete category to a thirdparty
+     *
+     * @param int		$id	Id of thirdparty
+     * @param array     $request_data   Request datas
+     *
+     * @return mixed
+     *
+     * @url POST {id}/deleteCategory
+     */
+    function deleteCategory($id, $request_data = NULL) {
+        if (!isset($request_data["category_id"]))
+            throw new RestException(400, "category_id field missing");
+        $category_id = $request_data["category_id"];
 
+      if(! DolibarrApiAccess::$user->rights->societe->creer) {
+			  throw new RestException(401);
+      }
+
+      $result = $this->company->fetch($id);
+      if( ! $result ) {
+          throw new RestException(404, 'Thirdparty not found');
+      }
+      $category = new Categorie($this->db);
+      $result = $category->fetch($category_id);
+      if( ! $result ) {
+          throw new RestException(404, 'category not found');
+      }
+
+      if( ! DolibarrApi::_checkAccessToResource('societe',$this->company->id)) {
+        throw new RestException(401, 'Access not allowed for login '.DolibarrApiAccess::$user->login);
+      }
+      if( ! DolibarrApi::_checkAccessToResource('category',$category->id)) {
+        throw new RestException(401, 'Access not allowed for login '.DolibarrApiAccess::$user->login);
+      }
+
+      $category->del_type($this->company,'customer');
+      return $this->company;
+    }
 
     /**
      * Get outstanding proposals of thirdparty