Polymer 3.0 support: Remove neon-animation dependency

[MajorBreakfast]

Neon animation is deprecated and there is no Polymer 3.0 support for it. Since iron-dropdown is widely used, it is very important that it starts working with Polymer 3.0. Shall I work on this and provide a PR?

[homerjonathan]

Starting to work on it.

[freshp86]

What is the status of this issue? Also, is it possible for the neon-animation dependency to be removed from the 2.x branch as well, for cases where Polymer2 is still being used?

cc @azakus @rbp314