Remove unused gtranslator-logo pixmap

No more used since the refactoring of the about dialog, i.e. commit
f270a263

This also removes the pixmaps subdirectory and its meson.build, as
there is nothing more to install.
1 job for remove-unused-pixmaps in 9 minutes and 51 seconds
Status Job ID Name Coverage
  Test
passed #898476
flatpak
flatpak

00:09:51