kbuild: dtbinst: remove unnecessary __dtbs_install_prep target

Since commit 5399eb9b ("dtbsinstall: don't move target directory
out of the way"), the target __dtbs_install_prep is invoked just for
creating the install directory, but all the necessary directories
are automatically created by:
   cmd_dtb_install = mkdir -p $(2); cp $< $(2)
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

scripts/Makefile.dtbinst

@@ -20,12 +20,6 @@ include include/config/auto.conf
 include scripts/Kbuild.include
 include $(src)/Makefile
 
-PHONY += __dtbs_install_prep
-__dtbs_install_prep:
-ifeq ("$(dtbinst-root)", "$(obj)")
-	$(Q)mkdir -p $(INSTALL_DTBS_PATH)
-endif
-
 dtbinst-files	:= $(dtb-y)
 dtbinst-dirs	:= $(dts-dirs)
 
@@ -35,8 +29,6 @@ quiet_cmd_dtb_install =	INSTALL $<
 
 install-dir = $(patsubst $(dtbinst-root)%,$(INSTALL_DTBS_PATH)%,$(obj))
 
-$(dtbinst-files) $(dtbinst-dirs): | __dtbs_install_prep
-
 $(dtbinst-files): %.dtb: $(obj)/%.dtb
 	$(call cmd,dtb_install,$(install-dir))