SPIKE: Remove "Open" button from PDF.js

PDF.js includes an icon in the toolbar for opening a new PDF in the viewer.

Using this feature while Hypothesis is active results in a confusing experience for the user:
- The new PDF is loaded in the viewer, but annotations from the previously shown PDF still display in the sidebar
- User can annotate in the new document, with new annotation added in the sidebar with the old

This is especially unwanted in the LMS app, where a instructor has assigned a specific document and there should not be any opportunity for a student to open a different document in the same viewer.

The issue identifies PDF.js's toolbar Open button and the Hypothesis sidebar behavior, but names no repository files, tests, or entry points. Start by locating the viewer toolbar configuration and checking the LMS viewer behavior; done means the Open control is unavailable and changing documents cannot retain annotations from the previous document.

Written by the indexing model from the issue text.