Med: xml: fix RELAG NG spec. violation in rule.rng

The violation is in allowing oneOrMoregroupattribute path in the XML
notation of the schema. And removing unnecessary constructs/reducing
complexity is a good thing anyway.

See [1] and the discussion [ML-thread:2,overview:3]; perhaps also [4].

[1] https://www.oasis-open.org/committees/relax-ng/spec-20011203.html#IDAPMBS
[2] http://blog.gmane.org/gmane.text.xml.relaxng.general/month=20040428
[3] http://www.dpawson.co.uk/relaxng/schema/schemaDes.html#d750e399
[4] http://www.relaxng.org/tutorial-20011203.html#IDA0FYR

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>