Revert "kbuild: Create directory for target DTB"

This reverts commit 77479b38.

Since commit 8a78756e ("kbuild: create object directories simpler
and faster"), all directories for 'targets' are created.

'mkdir -p $(dir ${dtc-tmp})' is no longer needed.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>