Fix changing values of cells of type "inlineStr"

[afalkenhahn]

When changing the value of a cell whose type is inlineStr, OpenXLSX doesn't remove the <is> node which contains the inlined string. This will break the XLSX document because it will now have both <is> and <v> nodes within the <c> node. Excel will no longer load such files. This pull request fixes the issue by removing the <is> node whenever a cell value is set or cleared.