internal/driver: remove an unused type Modifier

internal/driver/modifier.go

@@ -1,25 +0,0 @@
-// Copyright 2019 The Ebiten Authors
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//     http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-package driver
-
-type Modifier int
-
-const (
-	ModifierShift Modifier = 1 << iota
-	ModifierControl
-	ModifierAlt
-	ModifierCapsLock
-	ModifierNumLock
-)