All version control systems I am aware of have some form of "obliterate" capability that allows you to remove data permanently. This is normally not something you want to do lightly as it inevitably involves rewriting history. Most version control systems in fact restrict this ability to administrators or at least to users with direct access to the central repository. In a distributed version control system like git, this is complicated by the fact that many people may have clones of the repository and therefore copies of the sensitive data. If you cannot guarantee that all copies are destroyed, you have a problem.
Some general advice from an SCM pro: As a matter of policy I won't use any "obliterate" equivalent unless requested to do so by the company's legal department. This is because, as I said, you are rewriting history, and the history of the code in your repository can be considered legal evidence in some cases. I probably would recommend not removing a password as a password can be changed.
In your particular instance, I would strongly recommend that you not do anything without talking to your supervisor, even if you do have the access or the knowledge to remove the password. Messing around with the history in your repository is not something you should be doing on your own authority.