obs-ffmpeg: Remove unexisting class name declaration

The class `Logger` was declared but it is never defined nor used.

plugins/obs-ffmpeg/obs-ffmpeg-url.h

@@ -137,6 +137,5 @@ enum type {
 	debug = 7 //issue w/ libobs so LOG_DEBUG is removed
 };
 }
-class Logger;
 } // namespace srt_logging
 #endif