export.h: remove code for prefixing symbols with underscore

CONFIG_HAVE_UNDERSCORE_SYMBOL_PREFIX was selected by BLACKFIN, METAG.
They were removed by commit 4ba66a97 ("arch: remove blackfin port"),
commit bb6fb6df

 ("metag: Remove arch/metag/"), respectively.

No more architecture enables CONFIG_HAVE_UNDERSCORE_SYMBOL_PREFIX.
Clean up the export.h headers.  I am keeping VMLINUX_SYMBOL() and
VMLINUX_SYMBOL_STR() because they are widely used.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>