vp8enc: generating streamheader is now obsolete

Submitted by Thiago Sousa Santos @thiagossantos

Link to original bug (#726118)

Description

This is a followup for https://bugzilla.gnome.org/show_bug.cgi?id=722682

Now that oggmux can generate the streamheaders by itself this code can be removed from vp8enc.

The issue here is that vp8enc implements the tagsetter interface and those tags were being put on the streamheaders. What can be done? Push the tags that are set downstream? Oggmux should be modified to receive those tags and generate these tag headers so they are not lost.