net: macsec: remove the prepare flag from the MACsec offloading context

Now that the MACsec offloading preparation phase was removed from the
MACsec core implementation as well as from drivers implementing it, we
can safely remove the flag representing it.

Signed-off-by: Antoine Tenart <atenart@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>