Default FTN charset to CP437 (not ASCII) for QWK-imported messages

A QWKnet tagline usually contains CP437 character 254 and if/when appended to
a message during export, would "convert" the message to CP437 thus making the
"ASCII" charset advertisement incorrect.

If a FIDO CHARSET value was already specified (e.g. in HEADERS.DAT), that
value will (still) not be overridden. So the *original* import via QWK (before
it gets exported to a QWKnet) is where this change will take effect.

Fix for issue #741

src/sbbs3/qwktomsg.cpp

@@ -483,8 +483,6 @@ bool sbbs_t::qwk_import_msg(FILE *qwk_fp, char *hdrblk, uint blocks
 		const char* charset = FIDO_CHARSET_CP437;
 		if(msg->hdr.auxattr & MSG_HFIELDS_UTF8)
 			charset = FIDO_CHARSET_UTF8;
-		else if(str_is_ascii(body) && str_is_ascii(tail))
-			charset = FIDO_CHARSET_ASCII;
 		smb_hfield_str(msg, FIDOCHARSET, charset);
 	}