Revert "Ensure device tree DTS is compiled"

This reverts commit 27cb7300.

I am not sure if I correctly understood the log of commit 27cb7300
("Ensure device tree DTS is compiled"), but the code-diff looks like
it was trying to solve the missed re-compilation when .dts was modified.

Recently, commit 2737dfe0 ("kbuild: make arch-dtbs target PHONY")
fixed the issue in a more correct and more complete way.

Anyway, since the former commit, we see a clumsy log like this:

  make[2]: 'arch/sandbox/dts/sandbox.dtb' is up to date

Another problem is, it created multiple paths to descend into
arch/*/dts/, causing a race in parallel building.

So, let's revert it.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>