The commits I made to that fork were made months before the deletion of the license, so I should be legally fine. However, I'm curious as to the legal status of a repository that deleted its license. Am I legally able to commit to my fork? Do I have to delete my license.txt?
Open source licenses are irrevocable so therefore you can use any code which was at any point in time available under the MIT license - which in your case pretty clearly means anything up until the license was deleted from the repository. That code is available to you under the MIT license, so you must still comply with the conditions of the license; in particular, this means you must not delete your copy of the MIT license.
The code (if any) added to the repository after the deletion of the license file is not available for you to use.