Correct old cell name unbinding when renaming cell.

Correct old cell name unbinding when renaming cell.

Bug is to unbind old cell names when renaming a cell with
'makunbound'. when the old cell name is of A1 type, then
'kill-local-variable' must be used instead, so that only the current
spreadsheet is affected. When the old cell name is a renamed cell,
then 'ses--unbind-cell-name' must be used in order to remove the old
name from the name hashmap.

* ses.el (ses-rename-cell): check consistency of cell symbol from
text-property and from array object. Instead of 'makunbound', use
either 'ses--unbind-cell-name' or 'kill-local-variable' depending on
whether the cell old name is a named cell or an A1 type cell