Remove t:ctrlspace_start_window on CtrlSpace close

Allows to provide a statusline indicator of the active target
window when ctrlspace is active.

Example:

    " Add this to status line: %{CtrlSpaceStartWindowIndicator()}

    function! CtrlSpaceStartWindowIndicator()
      return gettabvar(tabpagenr(), "ctrlspace_start_window") == winnr() ? g:ctrlspace_symbols.ia : ""
    endfunction