Add noUnusedLocals tsconfig flag, fix issues

Two parts were commented out:
- _writeStopped: This is because _xoffSentToCatchUp references this variable
  and the feature is sort of half implemented/disabled.
- _clearChar: This is because some comments still reference this, it's part
  of the work needed to do better line redrawing.

src/renderer/atlas/CharAtlas.ts

@@ -6,7 +6,6 @@
 import { ITerminal } from '../../Types';
 import { IColorSet } from '../Types';
 import { ICharAtlasConfig } from './Types';
-import { isFirefox } from '../../shared/utils/Browser';
 import { generateCharAtlas, ICharAtlasRequest } from '../../shared/atlas/CharAtlasGenerator';
 import { generateConfig, configEquals } from './CharAtlasUtils';
 

src/renderer/atlas/CharAtlasUtils.ts

@@ -4,7 +4,6 @@
  */
 
 import { ITerminal } from '../../Types';
-import { ITheme } from 'xterm';
 import { IColorSet } from '../Types';
 import { ICharAtlasConfig } from './Types';