firmware-utils/ptgen: remove unused variable assignment

Discovered by coverty:

 CID 1473630:  Code maintainability issues  (UNUSED_VALUE)

 Assigning value from "type_to_guid_and_name(type, &name)" to
 "part_guid" here, but that stored value is overwritten before it can
 be used.

Remove the now redundant assignment of part_guid which is also set
conditionally later on.

Fixes: 4a078bd135 ("firmware-utils/ptgen: fix partition guid and name")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>

tools/firmware-utils/src/ptgen.c

@@ -630,7 +630,6 @@ int main (int argc, char **argv)
 			break;
 		case 't':
 			type = (char)strtoul(optarg, NULL, 16);
-			part_guid = type_to_guid_and_name(type, &name);
 			break;
 		case 'a':
 			active = (int)strtoul(optarg, NULL, 0);