config.mk: Delete unused variable BCURDIR

This variable was abolished by Kbuild series.
I forgot to delete it.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/config.mk b/config.mk
index 9897f20..e59697f 100644
--- a/config.mk
+++ b/config.mk
@@ -46,7 +46,5 @@
 CPPFLAGS = $(RELFLAGS)
 CPPFLAGS += -pipe $(PLATFORM_CPPFLAGS)
 
-BCURDIR = $(subst $(SRCTREE)/,,$(CURDIR:$(obj)%=%))
-
 LDFLAGS += $(PLATFORM_LDFLAGS)
 LDFLAGS_FINAL += -Bstatic