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>
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 03905aa..e6ce7b1 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -1,5 +1,3 @@
-# Backward compatibility - to be removed...
-extra-y	+= $(EXTRA_TARGETS)
 # Figure out what we need to build from the various variables
 # ===========================================================================
 
@@ -140,7 +138,7 @@
 #	target: source(s) FORCE
 #		$(if_changed,ld/objcopy/gzip)
 #
-#	and add target to EXTRA_TARGETS so that we know we have to
+#	and add target to extra-y so that we know we have to
 #	read in the saved command line
 
 # Linking