Remove "use external payload" flag

External payload is always necessary as all databases have a size limit
for binary blobs, thus payload parts beyond that need to be stored in
files.

Fixes all sorts of problems users with old config got into because
the value in their config files was "false" from early versions which
had this feature disabled as it wasn't fully developed/tested yet.

See bug 275561 for an instance where switching this value manually
fixed part of the user's problem.
REVIEW: 101703