These are no longer needed as the visual highlights in PDFs are now
created by SVG elements above the canvas, rather than by a background
color in PDF.js's text layer.

Removing these overrides fixes a cosmetic issue where overlapping text
boxes in the selection layer could create different shades of blue (or
whatever the selection color is) instead of blending into one, as shown
in https://github.com/hypothesis/private-issues/issues/108#issuecomment-1115392598.