Disable the creation of a default profile

In TB3: Tor Browser's official .mozconfigs., we set

imply_option("MOZ_DEDICATED_PROFILES", False)

This option makes us use a single profile for all our channels.

Personally, I do not like this option very much (but at this point it's part of our legacy, for Tor Browser). But what should we do with Base Browser/S131 browser? Should we move it to the base-browser mozconfig commit?

Without it, when we make Firefox create the actual profiles (e.g., with portable mode off), it creates a default profile for legacy versions, see also Bug 1770174.

We could put these lines between ifdef/endif when we don't want this behavior, while keeping the dedicated profiles.