qga: remove pointless 'blockrpcs_key' variable

qga: remove pointless 'blockrpcs_key' variable

This variable was used to support back compat for the old config
file key name, and became redundant after the following change:

  commit a7a2d636ae4549ef0551134d4bf8e084a14431c4
  Author: Philippe Mathieu-Daudé <philmd@linaro.org>
  Date:   Thu May 30 08:36:43 2024 +0200

    qga: Remove deprecated 'blacklist' argument / config key

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20240712132459.3974109-21-berrange@redhat.com>
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>