Bug 40002: Remove about:ion

Firefox Ion (previously Firefox Pioneer) is an opt-in program in which people
volunteer to participate in studies that collect detailed, sensitive data about
how they use their browser.

Bug 41662: Disable about:sync-logs

Even though we disable sync by default with
`identity.fxaccounts.enabled`, this about: page is still avilable.
We could throw an exception on the constructor of the related
component, but it would result only in an error in the console, without
a visible "this address does not look right" error page.
If we fix the issues with MOZ_SERVICES_SYNC, we can restore the
component.