csky: remove unused cmd_vdso_install

You cannot run this code because arch/csky/Makefile does not define the
vdso_install target.

It appears that this code was blindly copied from another architecture.

Remove the dead code.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Guo Ren <guoren@kernel.org>

arch/csky/kernel/vdso/Makefile

@@ -58,13 +58,3 @@ quiet_cmd_vdsold = VDSOLD  $@
 # that contains the same symbols at the same offsets.
 quiet_cmd_so2s = SO2S    $@
       cmd_so2s = $(NM) -D $< | $(srctree)/$(src)/so2s.sh > $@
-
-# install commands for the unstripped file
-quiet_cmd_vdso_install = INSTALL $@
-      cmd_vdso_install = cp $(obj)/$@.dbg $(MODLIB)/vdso/$@
-
-vdso.so: $(obj)/vdso.so.dbg
-	@mkdir -p $(MODLIB)/vdso
-	$(call cmd,vdso_install)
-
-vdso_install: vdso.so