Skip writing unranked beatmaps to osu_beatmap_difficulty_attribs

[smoogipoo]

A while back, we trimmed all attributes for unranked beatmaps from the osu_beatmap_difficulty_attribs table. This was done for space-saving reasons.

Without this PR, these attributes are still being written for all new beatmaps and for full reprocesses.

Of particular importance, note that:

• I haven't stopped writing of scoring attribs. It may be safe to not write them too, but I would rather not break anything there for now. This table is quite small anyway.
• This still writes to osu_beatmap_difficulty, where we may have decided to keep the star rating written to during the previous trimming.
• It now always queries osu_beatmaps, once for each beatmap. I'm hoping the overhead here is pretty minimal.

Haven't really tested this yet.