Preferences > Browse Packages
opens the Packages
folder; any folder that exists inside of that location is considered to be a package, and any resource files that appear inside of such a folder will apply.
Similarly, from that folder if you go up one folder to the parent, you will see a sibling Installed Packages
folder; any sublime-package
file that exists in that folder is a package and will be loaded.
That said, if you previously told Sublime to open all files with a specific extension as some other type of file, then it will remember that even if you removed the package; so you need to check your User
package for a sublime-settings
file that looks like it’s named the one that is causing you problems, and if you find it try moving it away to your Desktop or similar, then close and re-open the markdown file and see if the problem is gone.
Such file may also appear in the package you mentioned, if it did something janky like force a syntax on you just by having it installed (which is considered Bad Form ™).