um: Remove the call to SUBARCH_EXECVE1 macro

This macro has never been defined by any supported sub-architectures
in tree since it was introduced by commit 1d3468a6 ("[PATCH uml:
move _kern.c files").

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Signed-off-by: Richard Weinberger <richard@nod.at>