Convert from MDX to MDL. Then open with Notepad and you will see the animations nice and neatly placed in order. Simply go down the file and erase all the lines for the animations you do not want.
You could do that, but it would make no difference whatsoever. The filesize from animations comes not from the existence of the sequence, but from all the keyframes each sequence contains. You need Oinkerwinkle's Animation Remover, although I'm not sure where you could obtain the tool as I don't think it's on his website.
The bones. Or possibly helpers, maybe attachments... Many places. Just look for lines like this: XXXX: { x.xxxxxxx, x.xxxxxxx, x.xxxxxxxx }, With possibly a fourth x.xxxx for rotational values. Basically, you want to cut your filesize you need to remove all the keys in the sequence for every bone. Hence why I suggest Oinkerwinkle's Animation Remover. Fast and easy.