Webpack5 comments

[zoran995]

What this PR does

• Updates the versions of webpack plugins and cleanup no longer used ones.
• Resolves the Ion default token message issue. Depends on move default token message to Ion property cesium#88 being merge and release first

Test me

Locally:

• sync terriamap-terriajs dependencies
• install dependencies
• run the project
• check that everything works the same (type checking is still working, assimpjs works correctly (copy-plugin-webpack))

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated relevant documentation in doc/.
• I've updated CHANGES.md with what I changed.
• I've provided instructions in the PR description on how to test this PR.

[zoran995]

btn--left was an icon definition so it is safe to remove
btn-small wouldn't make any point here as we have all its styles defined/overriden here or in styled-component

[zoran995]

This is not used anywhere anymore