data: remove various historical images.

It is quite obvious that these were all kept around for the fun historical
background. And even though we can always get these back anytime through git
archeology, if these images disappear from our sight, we will just eventually
forget they even existed.

We now have a `gimp-data` repository for all data used and installed by GIMP. I
am not going to add these over there since it would go against the whole purpose
of splitting the source repository (we have too much data so we will want to
clone `gimp-data` as shallow so that it will only contain the necessary data to
install GIMP).
Yet I have created a page to reference and display all these original images
(and more! I spent quite some time researching old splashes and logos in our dev
log) for the pleasure of keeping the history of GIMP alive. It also reference
commit links for easy retrieval and further researches.

The only left images now are:

- gimp-logo.png and gimp-devel-logo.png: these are used in the About dialog or
  API reference. Ideally they should be generated from the SVG source.
- gimp-splash.png: used as splash image, in the Welcome dialog and in the
  Windows installer.
- wilber.png: only used in some Script-fu test scripts as far as I can see. This
  should also be generated from the same source SVG as gimp-logo.png, or maybe
  simply gimp-logo.png should be used instead.

Logos: https://gitlab.gnome.org/GNOME/gimp-data/-/blob/main/images/logo-log.md
Splashes: https://gitlab.gnome.org/GNOME/gimp-data/-/blob/main/images/splash-log.md

data/images/wilber-ui-shadow.svg

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