sb/amd/rs780: Remove dead assignment

We return immediately after, so this assignment can be removed.

Change-Id: I2a317ada0132a40b623402055b3b995bde80ddf5
Signed-off-by: Jacob Garber <jgarber1@ualberta.ca>
Found-by: scan-build 8.0.0
Reviewed-on: https://review.coreboot.org/c/coreboot/+/33773
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/southbridge/amd/rs780/cmn.c

@@ -263,7 +263,6 @@ u8 PcieTrainPort(struct device *nb_dev, struct device *dev, u32 port)
 		gfx_gpp_sb_sel = PCIE_CORE_INDEX_GPP;
 		break;
 	default:
-		gfx_gpp_sb_sel = -1;
 		return 0;
 	}