Based on this stackoverflow's answer.

To untrack a single file that has already been added/initialized to your repository, i.e., stop tracking the file but not delete it from your system use:

git rm --cached filename

To untrack every file that is now in your .gitignore:

First commit any outstanding code changes, and then, run this command:

git rm -r --cached .

This removes any changed files from the index(staging area), then just run:

git add .

Commit it:

git commit -m ".gitignore is now working"

To undo git rm --cached filename, use git add filename.