kbuild: remove unneeded mkdir for external modules_install

scripts/Makefile.modinst creates directories as needed.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -1779,10 +1779,8 @@ $(MODORDER): descend
 PHONY += modules_install
 modules_install: _emodinst_ _emodinst_post
 
-install-dir := $(if $(INSTALL_MOD_DIR),$(INSTALL_MOD_DIR),extra)
 PHONY += _emodinst_
 _emodinst_:
-	$(Q)mkdir -p $(MODLIB)/$(install-dir)
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modinst
 
 PHONY += _emodinst_post