build: Fix the Makefile rules for the GSettings schema

There's no need for the extra *.in indirection with intltool >= 0.50.1.
That's what commit 34093e8e, where the schema was introduced,
intended to do but somehow didn't end up doing.

The use of INTLTOOL_XML_RULE was completely needless. It is meant for
merging translations back into appdata and desktop files, but that's
not the case for GSettings schemas. Like code, those translations are
fetched at runtime.

Fallout from 34093e8e