modpost: remove symbol prefix support

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,
hence VMLINUX_SYMBOL_STR(foo) can be simplify replaced with "foo".

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