search: common Subject: normalization for Re: prefixes
Drop German ("Aw:") support since it's non-standard and
is not supported by Mail::Thread and non-English prefixes
are more likely to conflict with prefixes used in Free Software
development where ("subsection:") prefixes are common and English is the
common language.
Anyways we don't filter "Vs: " (Finnish) or "Sv: "
(Norwegian, Swedish, Danish, Icelandic), either.