[api] Remove duplicated code from request controller

There already is a before_action that makes sure a bs_request was found.
The code is of the before_action is even identical to the dropped code.

src/api/app/controllers/webui/request_controller.rb

@@ -145,12 +145,6 @@ def require_request
   end
 
   def changerequest
-    @req = BsRequest.find_by_number(params[:number])
-    unless @req
-      flash[:error] = "Can't find request #{params[:number]}"
-      redirect_back(fallback_location: user_show_path(User.current)) && return
-    end
-
     changestate = nil
     %w(accepted declined revoked new).each do |s|
       if params.has_key? s