[LIVY-983] Remove Glyphicons fonts and icons

[javiroman]

Removing Fonts and Icon sets from the website according to this:
https://issues.apache.org/jira/browse/LIVY-983

[ajbozarth]

Did you check they aren't used anywhere? I didn't look myself but noticed you only deleted the icons and didn't edit any css or html files.

[javiroman]

The site builds fine, and apparently the fonts and icons are not used, however this PR is not finalized because the bootstrap 3 (not the custom CSS style) it's trying to load the fonts/icons removed and we get a "no found" error in the browser console. So I have to update this PR (we can considered this PS a "draft").

To summarize for removing the Glyphicons components we have to upgrade from Bootstrap 3 to Bootstrap 4 (Glyphicons are embedded into Bootstrap 3), so the strategy is:

1. Upgrade the server UI to Bootstrap 4: https://github.com/apache/incubator-livy/tree/master/server/src/main/resources/org/apache/livy/server/ui/static (already done in my local repo, so 100% done)
2. Upgrade the API documentation to Bootstrap 4: https://github.com/apache/incubator-livy/tree/master/docs/assets/themes/apache (on-going right now, about a 75% done)
3. Upgrade to Bootstrap 4 the site (this PR): Pending right now.

So in a few days I guess I will have all of this work done for closing this issue.