qga: Remove trivial "Returns:" sections

qga: Remove trivial "Returns:" sections

The QAPI doc generator recently started to auto-generate return
documentation when there is no "Returns:" section (commit 636c96cd77d
"qapi: Fix undocumented return values by generating something").
Remove "Returns:" sections where the auto-generated text is obviously
no worse.  For instance, guest-info's documentation changes from

    Return:
       GuestAgentInfo -- GuestAgentInfo

to

    Return:
       GuestAgentInfo

The auto-generated returns all are in the exact same spot.

We did this for qapi/ in commit 0462da9d6b1 (qapi: remove trivial
"Returns:" sections).

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20250717115246.3830007-3-armbru@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>