Documentation: kbuild: Remove obsolete dtc_cpp section

Commit b40b25ff ("kbuild: always
run gcc -E on *.dts, remove cmd_dtc_cpp") improved the functionality of
cmd_dtc_cpp and merged it back into cmd_dtc.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>