kbuild: remove unused dtc-version.sh script

This is U-Boot own code, and no longer used since commit 36dd5f1b
("dtc: Switch to building and using our own dtc unless provided").

Prior to that commit, U-Boot relied on an external dtc, so this script
was used to check the dtc version.

Now U-Boot bundles our own dtc in script/dtc/dtc like Linux kernel.

Users are still allowed to pass DTC= option from the command line,
but they are supposed to choose correct version of dtc in this case.
So, we do not check the dtc version any more.

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