kbuild: delete SPLTREE and TPLTREE

These variable are no longer used.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/Makefile b/Makefile
index fa1e0ae..c8d5ccc 100644
--- a/Makefile
+++ b/Makefile
@@ -165,10 +165,6 @@
 
 export srctree objtree VPATH
 
-SPLTREE		:= $(objtree)/spl
-TPLTREE		:= $(objtree)/tpl
-export	SPLTREE TPLTREE
-
 MKCONFIG	:= $(srctree)/mkconfig
 export MKCONFIG