Remove iron-icons dependency

[rubenstolk]

It's not a good pattern to include all iron-icons when only one icon is needed.

[admwx7]

I agree @rubenstolk and will move iron-icons to being a optional dependency as part of the 2.0 release.

Once the Polymer team completes their tooling for resolving duplicated icons we might be able to move to a more elegant approach, in the meantime we'll just have to make due.