ignore MAC meta files on import and also remove any such files from the ...

...course

STUD-1725
@waheedahmed @symbolist @adampalay 
Note: you didn't follow the pattern I showed you. This query will not work with new assets. We moved the son from `_id` to `content_son` for new assets. I don't think mongo handles disjunctive queries very well; so, you'll probably need to do this as 2 separate queries. That is, wrap this whole function body in a
```
    for prefix in [`_id`, `content_son`]:
        query = SON([
            ('{}.tag'.format(prefix), XASSET_LOCATION_TAG),
            ('{}.category'.format(prefix), 'asset'),
            ('{}.name'.format(prefix), {'$regex': ASSET_IGNORE_REGEX}),
        ])
        ...
```