retire in-tree GPL dtc devicetree compiler

Now that we no longer have GCC 4.2.1 in the tree and can assume FreeBSD
is being built with a C++11 compiler available, we can use BSDL dtc
unconditionally and retire the GPL dtc.

GPL dtc now has FreeBSD CI support via Cirrus-CI to help ensure it
continues to build/work on FreeBSD and is available in the ports tree
if needed.

The copy of (copyfree licensed) libfdt that we actually use is in
sys/contrib/libfdt so the extra copy under contrib/dtc/libfdt can be
removed along with the rest of the GPL dtc.

Reviewed by: kevans, ian, imp, manu, theraven
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23192