Avoiding duplicate and allowing unset of codec parameter

Avoiding duplicate and allowing unset of codec parameter.

This allows unsetting Opus FEC or other codec parameter. This also avoids duplication.
I'd prefer not to have to use so many regexes. I think it would be best to have a routine that splits fmtp into a dictionary - first split based on semicolon, and then split all the x=y params into a {x: y}. Then, removeCodecParam can simply just remove x from the dictionary, and writing the dictionary out is trivial. AddCodecParam is similarly easy to implement as well.