Remove FLAGS constant

Only WebGL uses this

src/renderer/Constants.ts

@@ -1,17 +0,0 @@
-/**
- * Copyright (c) 2017 The xterm.js authors. All rights reserved.
- * @license MIT
- */
-
-/**
- * Flags used to render terminal text properly for the old CharData format
- */
-export const enum FLAGS {
-  BOLD = 1,
-  UNDERLINE = 2,
-  BLINK = 4,
-  INVERSE = 8,
-  INVISIBLE = 16,
-  DIM = 32,
-  ITALIC = 64
-}