ARROW-2689: [Python] Remove parameter timestamps_to_ms
ARROW-2689: [Python] Remove parameter timestamps_to_ms

This parameter is no longer existent. For the Parquet path it was replaced by `coerce_timestamps`, other cases should use `Column.cast()`.

Author: Korn, Uwe <Uwe.Korn@blue-yonder.com>

Closes #2129 from xhochy/ARROW-2689 and squashes the following commits:

38c5c481 <Korn, Uwe> ARROW-2689:  Remove parameter timestamps_to_ms

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