removing an x265 audio track without conversion?
Use ffmpeg*, a free command-line tool: 

ffmpeg -i input.mkv -c copy -map 0:v -map 0:a:0 singleaudio.mkv


where 0:a:0 is the index of the audio stream you wish to keep. In ffmpeg, numbering starts from zero, so 0:a:0 refers to the first audio stream in the source file. Running ffprobe input.mkv beforehand will show the contents of the input.

*download the latest snapshot/nightly build.