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: Masahiro Yamada <yamada.masahiro@socionext.com>