arm64 and riscv64 don't use mainbus_attach_args, remove

diff --git sys/arch/arm64/dev/mainbus.c sys/arch/arm64/dev/mainbus.c
index b3a7da20672..a9ac44882b4 100644
--- sys/arch/arm64/dev/mainbus.c
+++ sys/arch/arm64/dev/mainbus.c
@@ -29,7 +29,6 @@
 #include <dev/ofw/ofw_thermal.h>
 
 #include <arm64/arm64/arm64var.h>
-#include <arm64/dev/mainbus.h>
 
 int mainbus_match(struct device *, void *, void *);
 void mainbus_attach(struct device *, struct device *, void *);
diff --git sys/arch/arm64/dev/mainbus.h sys/arch/arm64/dev/mainbus.h
deleted file mode 100644
index 5f63d0ec459..00000000000
--- sys/arch/arm64/dev/mainbus.h
+++ /dev/null
@@ -1,27 +0,0 @@
-/* $OpenBSD: mainbus.h,v 1.2 2017/04/27 10:23:19 kettenis Exp $ */
-/*
- * Copyright (c) 2016 Patrick Wildt <patrick@blueri.se>
- *
- * Permission to use, copy, modify, and distribute this software for any
- * purpose with or without fee is hereby granted, provided that the above
- * copyright notice and this permission notice appear in all copies.
- *
- * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
- * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
- * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
- * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
- * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
- * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
- * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
- */
-
-#ifndef __MAINBUS_H__
-#define __MAINBUS_H__
-
-/* Passed as third arg to attach functions. */
-union mainbus_attach_args {
-	const char		*ma_name;
-	struct fdt_attach_args	 ma_faa;
-};
-
-#endif /* __MAINBUS_H__ */
diff --git sys/arch/riscv64/dev/mainbus.c sys/arch/riscv64/dev/mainbus.c
index b83626bb5b4..0c98e14f53c 100644
--- sys/arch/riscv64/dev/mainbus.c
+++ sys/arch/riscv64/dev/mainbus.c
@@ -27,7 +27,6 @@
 #include <dev/ofw/fdt.h>
 
 #include <machine/riscv64var.h>
-#include <riscv64/dev/mainbus.h>
 
 int mainbus_match(struct device *, void *, void *);
 void mainbus_attach(struct device *, struct device *, void *);
diff --git sys/arch/riscv64/dev/mainbus.h sys/arch/riscv64/dev/mainbus.h
deleted file mode 100644
index 020734479d7..00000000000
--- sys/arch/riscv64/dev/mainbus.h
+++ /dev/null
@@ -1,28 +0,0 @@
-/*	$OpenBSD: mainbus.h,v 1.2 2021/05/12 01:20:52 jsg Exp $	*/
-
-/*
- * Copyright (c) 2016 Patrick Wildt <patrick@blueri.se>
- *
- * Permission to use, copy, modify, and distribute this software for any
- * purpose with or without fee is hereby granted, provided that the above
- * copyright notice and this permission notice appear in all copies.
- *
- * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
- * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
- * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
- * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
- * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
- * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
- * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
- */
-
-#ifndef __MAINBUS_H__
-#define __MAINBUS_H__
-
-/* Passed as third arg to attach functions. */
-union mainbus_attach_args {
-	const char		*ma_name;
-	struct fdt_attach_args	 ma_faa;
-};
-
-#endif /* __MAINBUS_H__ */