session: handle unknown SSRC in FIR

https://bugzilla.gnome.org/show_bug.cgi?id=725712
rtpspeexdepay: Remove caps restrictions for depayloader

The "encoding-params" is optional in the SDP, because we now require
a subset of the caps, it would fail caps negotiatioin if it wasn't present.
So removed it from the template caps.
rtp: Remove caps restrictions from RTP depayloader sink caps

Remove caps restrictions that correspond to the default and are not
required in SDP. With the new usage of having pads require a subset
of the caps, they will make the negotiation fail.