kernel-build: No need to run clean and mrproper

The mrproper target cleans everything that clean does, no need to run
both.

Change-Id: I82e588157cf5d7ce50c2483aaefbfddc67d0ff84
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/kernel-build.sh b/kernel-build.sh
index 4b69270..24ee3ab 100755
--- a/kernel-build.sh
+++ b/kernel-build.sh
@@ -75,7 +75,6 @@
 powerpc64le-linux-gnu-gcc --version
 
 # Build kernel prep
-ARCH=powerpc CROSS_COMPILE=powerpc64le-linux-gnu- make clean
 ARCH=powerpc CROSS_COMPILE=powerpc64le-linux-gnu- make mrproper
 
 # Build kernel