qmp: Clean up qmp_query_spice() #ifndef !CONFIG_SPICE dummy

QMP command query-spice exists only #ifdef CONFIG_SPICE.  Due to QAPI
limitations, we need a dummy function anyway, but it's unreachable.

Our current dummy function goes out of its way to produce the exact
same error as the QMP core does for unknown commands.  Cute, but both
unclean and unnecessary.  Replace by straight abort().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
