AppArmor: drop the profile dedicated to Web Content processes.

AppArmor: drop the profile dedicated to Web Content processes.

Before Firefox 60, Web Content processes were instances of a dedicated
binary (plugin-container). But since Firefox 60, the Web Content processes are
instances of the very same executable as the parent Firefox process,
which makes it impossible to apply a different AppArmor policy to:

 - Web Content processes, that should ideally be more strictly confined

 - the new parent Firefox process that's spawned while restarting
   during a self-upgrade of Tor Browser

And indeed, we had to drop this distinction with commit
678d083491ceba5201d96b514173890944928540.

As a result, the new parent Firefox process that's spawned while restarting
during a self-upgrade of Tor Browser runs under the torbrowser_plugin_container
profile, i.e. more strictly confined than it should be, which breaks all kinds
of things.

A Firefox release manager tells me there's no plan to give Web Content processes
a dedicated binary again; let's give up and go back to confining the entire
browser under one single AppArmor profile, and rely on Firefox' own sandboxing
systems to protect itself against rogue Web Content processes.