mb/google/auron: Drop variant_romstage_entry

Replace it with `mainboard_post_raminit`.

Change-Id: I94636c775cee6c14317ecff36972e2d267d28c91
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/49774
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>

src/mainboard/google/auron/variant.h

@@ -9,7 +9,6 @@
 
 int variant_smbios_data(struct device *dev, int *handle,
 			unsigned long *current);
-void variant_romstage_entry(struct romstage_params *rp);
 void lan_init(void);
 
 void mainboard_fill_spd_data(struct pei_data *pei_data);