I do not advise following the info posted above.
First off, Slayers is for Modchips, not for softmods. If you try to wipe the drive with the wrong softmod you only screw things up worse and make it harder to fix. Slayers can be a great tool, but when used with softmods it can really screw you up if you pick a wrong option.
Secondly, SID's softmod is crap and nobody here recommends it. Krayzies Ndure v1.1 (+v1.1.1 update kit) is the best softmod in existence and by far the safest.
If you modded it years ago then you do have an old softmod, which Slayers will probably work with though.
You could just boot the game you modded it with (Spliter Cell you said) and select the option from the list to "restore to stock" or something along those lines.
You could use Slayers to wipe if you feel it's old enough.
You can get CASH v1.1 and wipe it and go back to stock also. It can wipe any softmod except SID 4.0 or higher.
Also there is a tutorial for removing softmods in the .xbe exploit section you can follow which is pretty much what you described with deleting things via FTP. Here it is, http://forums.xbox-scene.com/index.php?showtopic=426328