export.h: remove include/asm-generic/export.h

Commit 3a6dd5f6 ("riscv: remove unneeded #include
<asm-generic/export.h>") removed the last use of
include/asm-generic/export.h.

This deprecated header can go away.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

include/asm-generic/export.h

-/* SPDX-License-Identifier: GPL-2.0-only */
-#ifndef __ASM_GENERIC_EXPORT_H
-#define __ASM_GENERIC_EXPORT_H
-
-/*
- * <asm/export.h> and <asm-generic/export.h> are deprecated.
- * Please include <linux/export.h> directly.
- */
-#include <linux/export.h>
-
-#endif