kbuild: stop removing stale <linux/version.h> file

Revert commit 223c24a7 ("kbuild: Automatically remove stale
<linux/version.h> file").

It was more than 6 years ago. I do not expect anybody to start
git-bisect for such a big window.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -257,7 +257,6 @@ export building_out_of_srctree srctree objtree VPATH
 # of make so .config is not included in this case either (for *config).
 
 version_h := include/generated/uapi/linux/version.h
-old_version_h := include/linux/version.h
 
 clean-targets := %clean mrproper cleandocs
 no-dot-config-targets := $(clean-targets) \
@@ -1253,7 +1252,6 @@ endef
 
 $(version_h): FORCE
 	$(call filechk,version.h)
-	$(Q)rm -f $(old_version_h)
 
 include/generated/utsrelease.h: include/config/kernel.release FORCE
 	$(call filechk,utsrelease.h)