How do I remove the user registration/login form?

While it's technically possible to remove the login functionality, you shouldn't, because then your administrator could never log onto the site.

In order to remove the register functionality, you simply need to browse to admin/user/settings (for Drupal 6) or admin/config/people/accounts (for Drupal 7) and select the the "Only site administrators can create new user accounts" option.

While jhedstrom answer does work, I particuarly find that it doesn't fully address the issue.

Here's a simple way to remove the login form from the site on Drupal 7.

First, let's create a new simple page called Admin. Simply go to Content - Add Content - Basic Page. Enter whatever details you'd like such as title, body, etc. just make sure that under URL path settings you specify a common alias for adminstration, I used admin.

Second step, let's make sure that the login form only shows on the admin page we just created. Navigate to Structure - Blocks - User login (note tat the block name may vary from installation to installation), and hit Configure. Under Visibility settings select the option Only the listed pages so that the block only shows for the pages you specify on the input, and enter the alias you set on the step above (admin).

Finally, let's ensure that only administrators can create accounts, by following jhedstrom's suggestion. Go to Configuration - People - Account settings, then under Registration and cancellation, ensure that the box for Administrators Only is selected for the Who can register accounts? option.

I think this is a much nicer and detailed approach to ensure that the login form isn't displayed, considering you don't need people creating accounts on your site.