Remove IE9 hacks for the select's background image/padding?

[jdanyow]

Would the team accept a PR to undo #144? Reason I ask is the hacks cause problems with minified css. Here's a page exhibiting the problem (form-select is under the theme heading).

https://github.com/primer/primer/blob/ab720451c2e7e2ac4dfc7c208879b20ac2dbacc5/modules/primer-forms/lib/form-select.scss#L8-L10

[shawnbot]

Thanks for the heads up, @jdanyow. We don't support IE9 anymore, so as far as I'm concerned all of those hacks can go. Pull requests are certainly welcome! 🎉