kbuild: kill backward compatibility checks

These checks has been present for several kernel releases (> 5).
So lets just get rid of them.
With this we no longer check for use of:
EXTRA_TARGETS, O_TARGET, L_TARGET, list-multi, export-objs

There were three remaining in-tree users of O_TARGET in some
unmaintained sh64 code - mail sent to the maintainer + list.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>