[EuiIcon] Remove icons from the main elastic/eui package

The purpose of this ticket is to remove the old icon bundling, assets etc. from `@elastic/eui`.

## Acceptance criteria

- [ ] PR is marked as breaking change.
- [ ] Make sure there are no leftover usages in Kibana or Cloud UI.
- [ ] Make sure the migration is clear and covers all cases.
- [ ] Make sure it's communicated through email that this decision has been made.

Start by locating the old icon bundling and assets in the @elastic/eui package, then search Kibana and Cloud UI for remaining usages. Define the migration coverage and verify there are no leftover usages; completion also requires marking the PR as a breaking change and communicating the decision by email.

Written by the indexing model from the issue text.