How to remove solder mask in Eagle
Create a rectangle on the solder mask layer, which is a "negative" layer - anything you create there will not have soldermask.

Because the layer is negative and because the program stacks features, you can do this on the board, if it is application specific, rather than in the library part's footprint.

The advantage of doing it in the footprint (first copy or export the part to a custom library) is that you may have more ready access to the part geometry when defining it, and something that is a common need can stay with the part as a footprint variation.

Conversely, the advantage of doing it on the board is that what exactly you need may depend on the particular board you are designing.