modpost: remove leftover symbol prefix handling for module device table

Blackfin and metag were the only architectures that prefix symbols with
an underscore. They were removed by commit 4ba66a97 ("arch: remove
blackfin port"), commit bb6fb6df

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

It is no longer necessary to handle <prefix> part of module device
table symbols.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>