Reduce the amount of GStreamer stuff bundled with ntsc-rs

[valadaptive]

There's currently (on the order of magnitude of) ~100MB of GStreamer plugins shipped with ntsc-rs. This is a result of me copying everything in the GStreamer installation to the shipped downloads.

At least as of 1.26 (not sure if earlier), GStreamer's installer divides its plugins into several categories. There are lots of plugins we don't need (most video encoders, stuff for web streaming, etc). The hard part will be making sure I don't accidentally remove something important but subtle.