I've installed elementary many times and always had to install plex manually, but there is an easy way to remove it. First identify a file from plex, any file that is installed with plex would work. If you know where plex is installed his is easy, otherwise you may need to use find. Here is a good tutorial on find
Once you have the name run dpkg -S file to determin the name of the package that installed plex. For example:
dpkg -S /bin/cp
coreutils is the package that installed cp. So to remove it I'd run
sudo dpkg -r coreutils
Obviously, you don't want to remove coreutils, just whatever package installed plex.
This method will only work if plex was installed from a package. It won't work if plex was built from source.