Disable profile migration

Currently, we ship an override.ini that contains only EnableProfileMigrator=0, and only on macOS.

It disables the migration from other browser (as stated, for example, here):

EnableProfileMigrator is about displaying the migration from other browsers at startup.

So, I'm inclined to see it as something desirable for all our browsers. I think it is triggered when creating the profile, so only on macOS at the moment (I've tried to rename my profile directory, and remove the override.ini, but it didn't show me the prompt to migrate from other browsers...).

I haven't really checked what the migration does (e.g., if it only imports bookmarks), but the idea of keeping the browsers distinct is good, in my opinion. In any case, even with EnableProfileMigrator set to 0, the menu item to import bookmarks from other browsers remain available (I've tested on Linux, at the moment, because I cannot test on macOS at the moment).

In any case, we could disable the migrator at build time, too, with MOZ_PROFILE_MIGRATOR (in browser/confvars.sh). I prefer this solution, because it's yet another thing moved from the build repository to this one.