If you installed from snap, run:
sudo snap remove blender
If you installed from PPA, run:
sudo apt-get remove --autoremove blender
if you installed in /opt as you describe, run:
sudo rm -rf /opt/blender
When you install 2.8, you may wish to install with a package manager so it automatically updates:
sudo add-apt-repository ppa:thomas-schiex/blender
sudo apt-get update
sudo apt-get install blender