By checking the ".bsr" file path, by opening those said files with an editor of your choice, looking for each *.ddj, *.bmt, *.bms,... Of each viable file, and deleting everything else.
Same goes for music files, get in game, listen to the music, decide which one to delete, or check the text files about which region has been declared with which *.ogg file.
And so on.
You see already, it requires hell of a time and isn't worth the few megabytes you are going to save. You won't reduce the size by 100mb nor even by 50mb (if it's not an overly raped file cluster by someone who overedited that client and pasted every crap everywhere). The regular 1,8gb ish file size of Silkroad shouldn't bother you at all 😉