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>

arch/um/kernel/exec.c

@@ -35,8 +35,5 @@ void start_thread(struct pt_regs *regs, unsigned long eip, unsigned long esp)
 	PT_REGS_IP(regs) = eip;
 	PT_REGS_SP(regs) = esp;
 	clear_thread_flag(TIF_SINGLESTEP);
-#ifdef SUBARCH_EXECVE1
-	SUBARCH_EXECVE1(regs->regs);
-#endif
 }
 EXPORT_SYMBOL(start_thread);