gnome2*.eclass: Move the preinst conditional out of gnome2_schemas_update

gnome2*.eclass: Move the preinst conditional out of gnome2_schemas_update

Move the GNOME2_ECLASS_GLIB_SCHEMAS conditional from
gnome2_schemas_update straight into the implementation of gnome2.eclass
postinst/postrm.

This variable is set in preinst to indicate whether any files were
installed. However, the updater itself does not use the list in any way
and updates all the schemas anyway.

Therefore, avoid requiring the ebuilds to explicitly define
preinst/postinst when it is known that the package installs schemas,
and instead let gnome2_schemas_update called in postinst/postrm update
schemas unconditionally.