UML: remove unused cmd_vdso_install

You cannot run this code because arch/um/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: Richard Weinberger <richard@nod.at>