Avoid hardcoding sysconfdir

Just rely on %configure passing the correct value.

This fixes the build for flatpak modules where sysconfdir is not /etc,
but /app/etc.

udisks2.spec

@@ -262,7 +262,6 @@ sed -i udisks/udisks2.conf.in -e "s/encryption=luks1/encryption=%{default_luks_e
 autoreconf -ivf
 # modules need to be explicitly enabled
 %configure            \
-    --sysconfdir=/etc \
 %if %{with_gtk_doc}
     --enable-gtk-doc  \
 %else