mrc.sh: remove redundant extraction logic

the extract_archive function already provides use
of unzip, and works just fine (for mrc.bin extraction)

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/mrc.sh

@@ -19,7 +19,7 @@ extract_mrc()
 	x_ cd "${appdir}"
 	extract_partition
 	extract_shellball
-	extract_coreboot
+	extract_archive "${SHELLBALL}" .
 	) || err "mrc download/extract failure"
 
 	"${cbfstool}" "${appdir}/"bios.bin extract -n mrc.bin \
@@ -51,13 +51,3 @@ extract_shellball()
 	printf "cd /usr/sbin\ndump chromeos-firmwareupdate ${SHELLBALL}\nquit" \
 	    | debugfs "${ROOTFS}" || err "extract_shellball: debugfs"
 }
-
-extract_coreboot()
-{
-	_unpacked=$( mktemp -d )
-
-	printf "Extracting coreboot image\n"
-	[ -f "${SHELLBALL}" ] || \
-	    err "extract_coreboot: shellball missing in google cros image"
-	x_ unzip "${SHELLBALL}"
-}