Remove reference to no longer used variable chessboard_filter_attached in hook_uninstall()

function chessboard_uninstall() {
  variable_del('chessboard_filter_attached');
}

This variable was used in early 6.x-1.x-dev code to keep track of stylesheets that belonged to cached filtered text. The 6.x-1.x-beta1 release stores these structures in the exact same cache bin as the text.

Last chance to clean up your variables table with hook_uninstall()!