Delete unused CSS classes, variables, and theme properties.

[lindapaiste]

Changes:

1. Find class names in .scss files which are not used anywhere in the code and delete the styles for those classes.
2. Delete files help-modal.scss and collection-popover.scss entirely as they are not used.
3. Found a few themify variables which are no longer used.
4. Deleted SCSS variables which are not used in any files.

These steps were done in that order. For example, variable $lightsteelblue is used by the theme property console-info-background-color so it is only "unused" after removing that unused theme property.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

[raclim]

Thanks for working on this!