chore: Remove unused devDependencies

[pdehaan]

Removing eslint and shelljs since I couldn't see where they were used (for ESLint, the dependency is included with grunt-eslint anyways).

Here's the current output from depcheck, but everything else looks kosher since they're imported via babel or Grunt.

Although I'm now wondering if we need both flow-bin and grunt-flowbin.

$ depcheck

Unused devDependencies
* babel-istanbul-loader
* babel-plugin-transform-class-properties
* babel-plugin-transform-es2015-modules-commonjs
* babel-plugin-transform-flow-strip-types
* babel-preset-es2015
* babel-preset-stage-2
* flow-bin
* grunt-contrib-clean
* grunt-contrib-copy
* grunt-contrib-watch
* grunt-coveralls
* grunt-eslint
* grunt-flowbin
* grunt-mocha-test
* grunt-newer
* grunt-webpack
* mocha-multi
* webpack-dev-server

[kumar303]

Ah, yes, shelljs was used once but no longer is. It can be removed. I don't think we need a direct dep on eslint either, not sure why that was there. I don't think flow-bin needs to be a direct dep. The grunt-flowbin project was extracted from web-ext so that's probably why.

[pdehaan]

@kumar303 Want me to remove flow-bin devDep now, or leave that for a future PR?

[kumar303]

Sure, if you don't mind, you can remove flow-bin in this patch too

[pdehaan]

Done

[coveralls]

Coverage remained the same at 100.0% when pulling ca5f473 on pdehaan:unused-deps into 4cd13a2 on mozilla:master.

[coveralls]

Coverage remained the same at 100.0% when pulling b07dd28 on pdehaan:unused-deps into f6bad41 on mozilla:master.