Remove the pointer cursor on the table header when no sort or hints

## Expected Behavior
The data table shouldn't show a pointer styled cursor when hovering over the table's header if the columns don't have hints and are not sortable since there is nothing to click on and no info to show while hovering.

## Current Behavior

With no sorting on the table or hints for the headers, the cursor changes to the pointer style. Nothing happens when you click since there is no sorting.

## Steps to Reproduce (for bugs)

1. Create a table with no sorting.
2. Make sure there are no hints to be displayed.
3. Hover the cursor over a column header.

## Your Environment

| Tech | Version |
|--------------|---------|
| Material-UI | 3.x |
| MUI-datatables | 2.10.2 |
| React | 16.9.0 |
| browser | Chrome 76.x |

## Proposed change
https://github.com/gregnb/mui-datatables/pull/910
This simply changes the CSS to remove the pointer cursor when not sorting or using hints.