Absolutely.
Look before deleting though.
If you use release before going to SVN, the files in data/ULib get converted to a new format made for SVN.
There is no conversion from new to old, but, the SVN should have created a backup.
I forget the file names though. Might find some there.
If not, edit your /settings/users.txt as explained in the readme file and links around here for how to add yourself as superadmin in Garry's mod, then delete /data/ULib and restart your server.