* agent/agent.h (opt): Remove enable_extended_key_format.
* agent/gpg-agent.c (enum cmd_and_opt_values): Turn
oDisableExtendedKeyFormat and oEnableExtendedKeyFormat into dummy
options.
* agent/protect.c (do_encryption): Remove arg use_ocb and
corresponding code.
(agent_protect): Ditto.  Change all callers.
* agent/command.c (cmd_readkey): Do not test for key availability here
but defer that agent_write_shadow_key.
* agent/findkey.c (agent_write_private_key): Simplify due to the
removal of disable-extended-key-format.
(write_extended_private_key): Fold into agent_write_private_key.
Remove the maybe_update arg.
(agent_write_shadow_key): Ditto.  Simplify.