Remove Polymer references from installation instructions

[samiheikki]

Connects to https://github.com/vaadin/components-team-tasks/issues/420

I can create same PR to all components as well once this is merged

[web-padawan]

No need to submit PRs for each component. We usually review the PR in skeleton, and then push to master manually. Also, please add [skip ci] to commit message.

[samiheikki]

Added [skip ci] to commit msg

[lkraav]

Can you guys enlighten me what https://github.com/vaadin/components-team-tasks/issues/420 is all about? (private repo)

What's the usefulness of this change across components?

[samiheikki]

@tomivirkki can you share details for @lkraav

[web-padawan]

@lkraav the overall idea is that in future, we should iteratively move away from Polymer library (which is not recommended for using in new projects by Polymer team).

Until then, we would like to update docs so the users don't mistakenly associate Vaadin components with Polymer Elements, which are in the maintenance mode. I agree this change is rather a compromise but it also indicates that we at least acknowledge the need for updating our technology stack.