kconfig: remove silentoldconfig target

As commit 911a91c39cab ("kconfig: rename silentoldconfig to
syncconfig") announced, it is time for the removal.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
diff --git a/scripts/kconfig/Makefile b/scripts/kconfig/Makefile
index 5d37a60..63b6092 100644
--- a/scripts/kconfig/Makefile
+++ b/scripts/kconfig/Makefile
@@ -68,14 +68,7 @@
 $(simple-targets): $(obj)/conf
 	$< $(silent) --$@ $(Kconfig)
 
-PHONY += silentoldconfig savedefconfig defconfig
-
-# We do not expect manual invokcation of "silentoldcofig" (or "syncconfig").
-silentoldconfig: syncconfig
-	@echo "  WARNING: \"silentoldconfig\" has been renamed to \"syncconfig\""
-	@echo "            and is now an internal implementation detail."
-	@echo "            What you want is probably \"oldconfig\"."
-	@echo "            \"silentoldconfig\" will be removed after Linux 4.19"
+PHONY += savedefconfig defconfig
 
 savedefconfig: $(obj)/conf
 	$< $(silent) --$@=defconfig $(Kconfig)