Removed unused JavaScript files

What does this MR do and why?

Removes two JavaScript files which are loaded site-wide but do not actually do anything:

• tables.js: Adds a CSS class to tables that exceed the width of their parent node. This CSS class (fixed-table) that this adds is not referenced in any Docs CSS code and has no affect on the appearance of the table.
• toggle_popover.js: Toggles the visibility of Bootstrap popover components. We do not use any popovers on the Docs site.

This is one more step towards completing #439 (closed).

Screenshots, screen recordings, or links to review app

No visual changes.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

You can verify these aren't used by compiling the site and searching for strings that would be present if these scripts were in-use:

• make compile
• grep -rn "fixed-table" public/ (we would see this if the class added by tables.js was used by CSS code)
• grep -rn "data-toggle=\"popover" public/ (we would see this if we had an element that was used as a popover toggle)

The searches should not return any matches.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.