window: drop unused variable

Discovered with Clang’s scan-build tool:

    ../../../src/fr-window.c:3301:3: warning: Value stored to 'archive' is never read [deadcode.DeadStores]
                    archive =  private->archive_file;
                    ^          ~~~~~~~~~~~~~~~~~~~~~

The reference was removed with statusbar https://gitlab.gnome.org/GNOME/file-roller/-/commit/fd4d0c0af39bbdbbf68df65cccfe38b63ac19185