Remove "Vetted" param

Our VOEvents contain the following param:

<Param name="Vetted" dataType="int" value="0" ucd="meta.number" unit="">
<Description>
Indicates whether this candidate has undergone basic vetting by humans
</Description>
</Param>

However, this is redundant because we define preliminary alerts as alerts that have not undergone human vetting whereas initial, update, and retraction alerts have been vetted.

I suggest removing this flag. If we want to preserve the human-readable comment, then it can go inside <Why><Description></Description></Why>.

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