Remove redundant clean of Linux

The 'clean' target performs a subset of operations of the 'mrproper'
target. Calling 'clean' before 'mrproper' is therefore redundant.

Change-Id: Id600c99d7b5d9546578c7e5f40c375b187706dc5
diff --git a/Makefile b/Makefile
index d66164b..a1f3aa4 100644
--- a/Makefile
+++ b/Makefile
@@ -172,7 +172,6 @@
 
 .PHONY: linux_clean
 linux_clean:
-	+ $(LINUX_MAKE) clean
 	+ $(LINUX_MAKE) mrproper