dashenc: Don't output frameRates at both AS and Representation-level. Only output maxFrameRate at AS-level.

Fix conformance regarding section "3.2.4. Presence of Attributes and
Element" of the "Guidelines for Implementation: DASH-IF Interoperability
Points V4.1" (http://dashif.org/guidelines/)
Signed-off-by: Anton Schubert <ischluff@mailbox.org>
Signed-off-by: Peter Große <pegro@friiks.de>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>