hmmm, I think U should remove some "<th></th>" from ranking tmpl....
but if U don't wanna corrupt other ranking table, U should create extension for that purpose.
1. remove some code from tmpl/ranking_heading.php
2. copy this one file to extensions/noheadrank/views/ranking/tmpl/ranking_heading.php
3. disable this extension in other project
4. enjoy your first working extension
5. share your work to other users, maybe someone else need this great developing stuff