[BUG] Dynamic field schema methods erroneously declare subfields of Polyline points

[brimoor]

The snippet below:

import fiftyone as fo

dataset = fo.Dataset()
dataset.add_sample(
    fo.Sample(
        filepath="image.jpg",
        shapes=fo.Polylines(
            polylines=[
                fo.Polyline(
                    tags=['tag1', 'tag2'],
                    label='label1',
                    points=[[(0, 0), (0, 1), (1, 1), (1, 0), (0, 0)]]
                ),
            ]
        )
    )
)

dataset.get_dynamic_field_schema()

erroneously detects the following dynamic fields:

{
    'shapes.polylines.points[]': <fiftyone.core.fields.ListField object at 0x7f938808ac10>,
    'shapes.polylines.points[][][]': <fiftyone.core.fields.IntField object at 0x7f936824cdc0>,
}

I believe the fix here is that default list fields should not be inspected.