Update installer to removal of images

* src/nsis/inst-kleopatra.nsi, src/nsis/uninst-kleopatra.nsi: Don't
install/uninstall some images anymore.

The gpg4win images are now embedded in the executable.

The kleopatra_splashscreen.* and kleopatra_wizard.* images have been
removed. They haven't been used by Kleopatra for ages.