Boolean rewrite could remove more MatchAllDocsQuery filters

It seems that BooleanQuery rewrites will remove a MatchAllDocsQuery 'filter' clause only if there is at least one 'must' clause. I think we could also remove the 'filter' clause if there are no 'must' clauses, but at least one other 'filter' clause.

This could let queries like #field:value #: be rewritten to #field:value.