refactor: simplify public/images/banner.svg for easier manual editing

[mskiptr]

This removes about 20 KiB of unnecessary fluff from the SVG banner logo (out of ~40 KiB in total). None of that should change the visual appearance of the logo, though.

To ease the review process I strived to keep every commit narrowly focused. In cases where the default line diffs were not readable, I also included examples of git commands that make the actual change clearly visible.