um: Remove obsoleted declaration for execute_syscall_skas

The execute_syscall_skas() have been removed since
commit e32dacb9 ("[PATCH] uml: system call path cleanup"),
and now it is useless, so remove it.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Richard Weinberger <richard@nod.at>