RemoteLinuxRunconfiguration: Remove disabledReason()/setDisabledReason()

setDisabledReason() is never called so disabledReason() always returns
QString(). The base class implementation can handle that job, too.

Change-Id: I9f3f7cad2e8abc39f0d5001fa44506d81fe47e44
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>