osd: remove magical tmap -> omap conversion

This is incomplete and unfortunately unusable in its current state:

 - it would only set USES_TMAP for old encoded object_info_t and tmapput,
   but would NOT set it for tmapup
 - a config option turned that off by default.

That means that the mds conversion from tmap -> omap won't be able to use
this because any existing cluster has tmap objects without the USES_TMAP
flag set.  And we don't want to unconditionally try a tmap->omap conversion
on omap operations because there are lots of existing librados users out
there that will be negatively impacted by this.

Instead, the MDS will need to handle this conversion on the client side by
reading either tmap or omap objects and explicitly rewriting the content
with omap (while truncating the tmap data away).

The auto-conversion function was added in v0.44.

Signed-off-by: Sage Weil <sage@inktank.com>