I'm not sure there is much you CAN remove. The majority of what isn't needed isn't there. That stuff is placed in the Media Pack.
If you are comfortable with editing the XML you could go through and remove references to a large chunk of sound files and the few videos which are in the game without the media pack, then pluck them out individually. But I have a feeling that the PAK file is compressed well enough that you wouldn't gain anything from it unless you were able to form a new PAK file with just the parts you want to keep once you finished trimming it down.
EDIT: If you have Warlords AND BtS installed, you could uninstall both of them, then re-install just BtS to cut back on some space. (Can't just remove Warlords though, since that takes some files which BtS didn't install after detecting that you have Warlords)