kbuild: remove 'Using ... as source for kernel' message

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Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -1115,7 +1115,6 @@ PHONY += prepare archprepare prepare3
 # 1) Check that make has not been executed in the kernel src $(srctree)
 prepare3: include/config/kernel.release
 ifdef building_out_of_srctree
-	@$(kecho) '  Using $(srctree) as source for kernel'
 	$(Q)if [ -f $(srctree)/.config -o \
 		 -d $(srctree)/include/config -o \
 		 -d $(srctree)/arch/$(SRCARCH)/include/generated ]; then \