doc: Scute can automatically start the agent.

* README: Remove paragraph about the need to have an agent up and
running and the GPG_AGENT_INFO variable set.
* doc/manual/scute.texi: Likewise.

As all other components of GnuPG, Scute does not need the
GPG_AGENT_INFO variable anymore, and can start the agent on
demand.

• Signed-off-by: Damien Goutte-Gattat <dgouttegattat@incenp.org>