extindex: avoid invalid blobs after unref

extindex: avoid invalid blobs after unref

When unref-ing a blob from xref3, make sure the "preferred"
smsg->{blob} doesn't point to the blob we just unrefed.  This
is necessary because we periodically checkpoint our extindex
process to allow -watch and -mda processes to run.

This also gets rid of a lot of redundant code for ->remove_xref3,
since it's all handled in ExtSearchIdx, now.