AVFormatWriter: Cleanup GET_CONTEXT_DEFAULTS macro

* av_get_context_default(1,2,3) are deprecated. The functions are supposed
to "reset" a codec context to a clean state so that it can be opened again.
It was used to allow using AVStream.codec as a decoding context
That behaviour is now deprecated, so that's why the function is too.
* hint: The code is probably still doing something bad in that regard.
* Unfortunately the macro removal did end up in the codecparam diff,
but was originally intended to be a commit alone I can't rewrite history
unfortunately, at least an explanation is given here. In any case the
code wasn't used so no functional change, this is mostly educated cleanup.

src/add-ons/media/plugins/ffmpeg/AVFormatWriter.cpp

@@ -48,8 +48,6 @@ static const size_t kIOBufferSize = 64 * 1024;
 	// TODO: This could depend on the BMediaFile creation flags, IIRC,
 	// they allow to specify a buffering mode.
 
-#define GET_CONTEXT_DEFAULTS 0
-
 typedef AVCodecID CodecID;
 
 // #pragma mark - AVFormatWriter::StreamCookie