#17 Remove the deprecated string returning TCheckListBox_GetChecked

[Thomas Mueller]

The overload TCheckListBox_GetChecked(_clb; _IncludeDisabled): string in src/u_dzVclUtils.pas is deprecated in favour of TCheckListBox_GetCheckedArr, which returns a TStringArray; its TODO regrets ever returning a string. It could be removed once no caller is left.

From the TODO scan in issue 12 of dzlib's ISSUES.md.