Remove need for include Mongoid::Attributes::Dynamic

[sahglie]

doc.unset(field) in mongoid >= 7.x interprets this as setting an attribute and since there is no attribute called "#{key}.#{digest}" it fails. Previous version of mongoid appear to just update the hash that #{key} pointed to. So, looks like we just need to remove the cached digest from the hash using a different method. This would do it:

if (doc[k].try(:[], digest).try(:[], :count)).to_i > 1
  doc.inc("#{field}.count" => -1)
else
  h = doc[k] || {}
  h.delete(digest)
  doc.set("#{k}": h)
end

With this update and removing include Mongoid::Attributes::Dynamic from Problem class the entire test suite passes.