kbuild: remove unneeded mkdir for external modules_install

[ Upstream commit 4b97ec0e ]

scripts/Makefile.modinst creates directories as needed.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Stable-dep-of: c7b98de7 ("phy: qcom-qmp-combo: fix runtime suspend")
Signed-off-by: Sasha Levin <sashal@kernel.org>