Deleting columns does not adjust column dimensions

[Phrogz]

Steps to reproduce:

1. Create a workbook in Excel with varying column widths
   
2. Open in openpyxl
3. Use worksheet.delete_cols() to remove a column (e.g. column C in above)
4. Save the workbook and open in Excel

Expected: deleting column(s) adjusts the column_dimensions to match, as in Excel:

Actual: the original column dimensions are retained while the content is shifted:

[Phrogz]

Workaround: before you modify columns, set up a list of widths. Every time you delete column(s) or insert them, manually mutate this list. At the end, re-set all the widths.

Workaround: make wrapper functions for delete_cols and insert_cols, which shift all the widths in the column_dimensions map manually every time. (ick)

Feels like this bug came about because the implementation uses a mapping instead of a list, so it's "hard"/expensive to update on each column mutation.