Remove references to removed DISTRIBUTION_TEXT variable

* src/Makefile.am (gpg4win_pkg_kleopatra_configure,
gpg4win_pkg_kxmlgui_configure): Remove configure option referencing
DISTRIBUTION_TEXT.
* src/appimage/Makefile.am (appimage_pkg_kxmlgui_configure): Ditto.

DISTRIBUTION_TEXT was removed from configure.ac with revision 46438040.

Moreover, support for XMLGUI_DISTRIBUTION_TEXT (which appended this
string to the version number in KDE's bug reporting) has been removed
from kxmlgui with version 5.102. That's no loss for us because it was
only used for bugs reported to a different address than bugs.kde.org.
For Gpg4win we use bugs.kde.org and for other products we don't use
KDE's bug reporting functionality.