Retire ZFS.RELEASE file

Retire ZFS.RELEASE file

The ZFS.RELEASE file was originally added to document which
version of OpenSolaris the ZoL code was based on.  However,
that's no longer particularly important or useful.  We'll
likely never see a new onnv_* drop from Solaris, and even
if we do the ZoL changes are now extensive enough they
could not be easily applied.  We now treat Illumos as the
official upstream and cherry pick the patches we need.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>