Cell paragraphs that use `\plain` after `\intbl`, together with a single
`\trowd` and `\row` row terminators, should produce a flat table rather
than deeply nested tables.

```
% pandoc -f rtf -t native
{\rtf1\ansi\deff0
{\fonttbl{\f0 Times;}{\f1 Times;}{\f2 Times;}}
\trowd\trgaph60\trleft0
\clvertalt\cellx3245
\clvertalt\cellx6490
\clvertalt\cellx9735
\pard\intbl\s0\ql\plain\f0\fs20\plain\f2\fs22 A\plain\f1\fs22\cell
\pard\intbl\s0\ql\plain\f0\fs20\plain\f2\fs22 B\plain\f1\fs22\cell
\pard\intbl\s0\ql\plain\f0\fs20\plain\f2\fs22 C\plain\f1\fs22\cell
\intbl\row
\pard\intbl\s0\ql\plain\f0\fs20\plain\f2\fs22 1\plain\f1\fs22\cell
\pard\intbl\s0\ql\plain\f0\fs20\plain\f2\fs22 2\plain\f1\fs22\cell
\pard\intbl\s0\ql\plain\f0\fs20\plain\f2\fs22 3\plain\f1\fs22\cell
\intbl\row
}
^D
[ Table
    ( "" , [] , [] )
    (Caption Nothing [])
    [ ( AlignDefault , ColWidthDefault )
    , ( AlignDefault , ColWidthDefault )
    , ( AlignDefault , ColWidthDefault )
    ]
    (TableHead ( "" , [] , [] ) [])
    [ TableBody
        ( "" , [] , [] )
        (RowHeadColumns 0)
        []
        [ Row
            ( "" , [] , [] )
            [ Cell
                ( "" , [] , [] )
                AlignDefault
                (RowSpan 1)
                (ColSpan 1)
                [ Para [ Str "A" ] ]
            , Cell
                ( "" , [] , [] )
                AlignDefault
                (RowSpan 1)
                (ColSpan 1)
                [ Para [ Str "B" ] ]
            , Cell
                ( "" , [] , [] )
                AlignDefault
                (RowSpan 1)
                (ColSpan 1)
                [ Para [ Str "C" ] ]
            ]
        , Row
            ( "" , [] , [] )
            [ Cell
                ( "" , [] , [] )
                AlignDefault
                (RowSpan 1)
                (ColSpan 1)
                [ Para [ Str "1" ] ]
            , Cell
                ( "" , [] , [] )
                AlignDefault
                (RowSpan 1)
                (ColSpan 1)
                [ Para [ Str "2" ] ]
            , Cell
                ( "" , [] , [] )
                AlignDefault
                (RowSpan 1)
                (ColSpan 1)
                [ Para [ Str "3" ] ]
            ]
        ]
    ]
    (TableFoot ( "" , [] , [] ) [])
]
```