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

Commit 3a6dd5f ("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>
Link: https://lore.kernel.org/r/20240323090615.1244904-1-masahiroy@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Author: masahir0y