This was added as part of bug 360184 but no justification was given
for the "local-only" part. My Spidey sense tells me it was a hack-
around for the old implementation's tendency to freeze the UI while
searching for a photograph. So the "local-only" option really just
meant "don't freeze the UI for very long, please".
The new EPhotoCache-based implementation in 3.8 NEVER freezes the UI,
so the "local-only" option is no longer needed. If a remote address
book is slow or unresponsive we simply cancel the async photo lookup
when the user moves on to another email.
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