export.h: remove defined(__KERNEL__), which is no longer needed

The conditional, define(__KERNEL__), was added by commit f2355416
("export.h: allow for per-symbol configurable EXPORT_SYMBOL()").

It was needed at that time to avoid the build error of modpost
with CONFIG_TRIM_UNUSED_KSYMS=y.

Since commit b2c5cdcf ("modpost: remove symbol prefix support"),
modpost no longer includes linux/export.h, thus the define(__KERNEL__)
is unneeded.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Nicolas Pitre <nico@fluxnic.net>