docs: kbuild: remove mention to dropped $(objtree) feature

Commit 8d613a1d ("kbuild: drop $(objtree)/ prefix support
for clean-files") dropped support for prefixing with $(objtree).

Thus update the documentation to match that change.

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