libceph: remove dead code in osd_req_encode_op()

In osd_req_encode_op() there are a few cases that handle osd
opcodes that are never used in the kernel.  The presence of
this code gives the impression it's correct (which really can't
be assumed), and may impose some unnecessary restrictions on
some upcoming refactoring of this code.

So delete this effectively dead code, and report uses of the
previously handled cases as unsupported.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>