doc: Remove redundant command for quick start preflight

Signed-off-by: Christophe Courtaut <christophe.courtaut@gmail.com>

This patch removes a few commands, which are redundant with following
commands.

Btw, they are also unrelated with the subtitle of the section, which is
"Add the release key".
Merge pull request #839 from kri5/master

doc: Remove redundant command for quick start preflight

Reviewed-by: Sage Weil <sage@inktank.com>
Merge git://github.com/ceph/ceph