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>

scripts/Makefile.build

@@ -55,31 +55,6 @@ _dummy := $(shell [ -d $(obj) ] || mkdir -p $(obj))
 _dummy := $(foreach d,$(obj-dirs), $(shell [ -d $(d) ] || mkdir -p $(d)))
 endif
 
-
-ifdef EXTRA_TARGETS
-$(warning kbuild: $(obj)/Makefile - Usage of EXTRA_TARGETS is obsolete in 2.6. Please fix!)
-endif
-
-ifdef build-targets
-$(warning kbuild: $(obj)/Makefile - Usage of build-targets is obsolete in 2.6. Please fix!)
-endif
-
-ifdef export-objs
-$(warning kbuild: $(obj)/Makefile - Usage of export-objs is obsolete in 2.6. Please fix!)
-endif
-
-ifdef O_TARGET
-$(warning kbuild: $(obj)/Makefile - Usage of O_TARGET := $(O_TARGET) is obsolete in 2.6. Please fix!)
-endif
-
-ifdef L_TARGET
-$(error kbuild: $(obj)/Makefile - Use of L_TARGET is replaced by lib-y in 2.6. Please fix!)
-endif
-
-ifdef list-multi
-$(warning kbuild: $(obj)/Makefile - list-multi := $(list-multi) is obsolete in 2.6. Please fix!)
-endif
-
 ifndef obj
 $(warning kbuild: Makefile.build is included improperly)
 endif

scripts/Makefile.clean

@@ -40,7 +40,7 @@ subdir-ymn	:= $(addprefix $(obj)/,$(subdir-ymn))
 # build a list of files to remove, usually releative to the current
 # directory
 
-__clean-files	:= $(extra-y) $(EXTRA_TARGETS) $(always) \
+__clean-files	:= $(extra-y) $(always)                  \
 		   $(targets) $(clean-files)             \
 		   $(host-progs)                         \
 		   $(hostprogs-y) $(hostprogs-m) $(hostprogs-)

scripts/Makefile.lib

-# Backward compatibility - to be removed...
-extra-y	+= $(EXTRA_TARGETS)
 # Figure out what we need to build from the various variables
 # ===========================================================================
 
@@ -140,7 +138,7 @@ $(obj)/%:: $(src)/%_shipped
 #	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