I had the same issue, I fixed it by uninstalling Ubuntu Dock.
Just run sudo apt purge gnome-shell-extension-ubuntu-dock and after restarting gnome-shell you're good to go. Since there's no point in having both installed/enabled, this is a safe approach.

This would also uninstall the Ubuntu desktop meta package.
Not sure if this is a good thing to do :-/

~$ sudo apt purge gnome-shell-extension-ubuntu-dock
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages will be REMOVED:
  gnome-shell-extension-ubuntu-dock* ubuntu-desktop*

It's safe to remove the ubuntu-desktop package. Like you mentioned, it's just a meta package that holds a lot of dependencies when installing Ubuntu. It's only useful if you're planning on upgrading from 18.04 to 18.10 or any future version when they are released. I never update Ubuntu this way, always a fresh install, so the package is irrelevant.

EDIT: You can also read about it being safe to uninstall here: AskUbuntu: Is it safe to remove ubuntu-desktop package?

I took @fcastilloec's advice; it required a restart but I think the dock is finally gone in Activities Overview. :D

@fcastilloec thx! It's work for me.

@fcastilloec Thanks! It works for me too

Closing this then.

@fcastilloec Is there a way to reinstall the ubuntu-desktop meta package if you would want to?

@na21 yes, just type sudo apt install ubuntu-desktop on a command line and you're good to go. But this will install all of its dependencies, including gnome-shell-extension-ubuntu-dock which is what was causing this bug.