ObjectCacher: fix flush_set when no flushing is needed

C_GatherBuilder takes ownership of the Context we pass it. Deleting it
in flush_set after constructing the C_GatherBuilder results in a
double delete.

Fixes: #3946
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Sam Lang <sam.lang@inktank.com>