Revert incorrect change to function definition

The parameters for function excel_format_custom_field() were unduly
changed, preventing proper exporting of custom field data to Excel
(custom fields would be displayed as '@@'). This is a regression from
a39a948a439974b208cf7441d003177fc9d08105. Thanks to Roland Becker for
catching this.

This commit also fixes an incorrect comment in the function header for
excel_format_plugin_column_value().

Affects 0013728