How to Merge the Last Sub-Column with its parent Column

There should be at least two (2) Sub-Columns below a Column. 

To merge the last Sub-Column into the Column and clean up the visual clutter:

Step 1:  Click the Edit Board Layout button in the green right side bar

Step 2: Click the horizontal rectangular button between the Column and Sub-Column.

Step 3:  Click Save button in the right side bar.

The cards in the Sub-Column will be moved into the Column and the history of the Sub-Column will also be moved into and adopted by the Column above.
After the merge, you may want to edit the Column name.

Alternatively, if when removing the  Sub-Column, you want either the cards in the Sub-Column or the Sub-Colum history to be allocated to a different Column, Archive the Sub-column and a popup will enable the nomination of a different Sub/Column.