[JS] Remove hard-coded Field.nullable

[asfimport]

Context: #4502 (comment)

This isn't a huge issue since we can just elide validity buffers when null count is zero, but sometimes it's desirable to be able to assert a Field is never null.

Reporter: Brian Hulette / @TheNeuralBit

_{Note: This issue was originally created as ARROW-5689. Please see the migration documentation for further details.}

[domoritz]

Fields can be nullable or not now

const s = new Schema([new Field("foo", new Int32, false), new Field("bar", new Int32, true)]);