mac_oldworld: Drop a variable, use get_system_memory() directly

mac_oldworld: Drop a variable, use get_system_memory() directly

Half of the occurances already use get_system_memory() directly
instead of sysmem variable, convert the two other uses to
get_system_memory() too which seems to be more common and drop the
variable.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <b4c714e03690deb6f94f80f7a5b2af47d90550ae.1602805637.git.balaton@eik.bme.hu>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>