libceph: nuke bogus encoding version check in osdmap_apply_incremental()

Only version 6 of osdmap encoding is supported, anything other than
version 6 results in an error and halts the decoding process.  Checking
if version is >= 5 is therefore bogus.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>