flacdec: Add support for variable block size files and remove dead code

This dead code wasn't used since the 1.0 port and would need to
be modified heavily for variable block size support.

https://bugzilla.gnome.org/show_bug.cgi?id=729894
matroskamux: fix the memory leak of language attribute

https://bugzilla.gnome.org/show_bug.cgi?id=728418
rtpsession: reset state after scheduling BYE

After we do RTCP, we are not scheduling bye anymore.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=728714