Remove internals.settings.setDialogElementEnabled

It's not needed because dialog is enabled by default for layout tests,
and switching dialog on/off between tests probably doesn't work as
expected (see "IDL/bindings question" on blink-dev, and ClusterFuzz
found a crash using it)

BUG=260284

Review URL: https://chromiumcodereview.appspot.com/19537009

git-svn-id: svn://svn.chromium.org/blink/trunk@154817 bbb929c8-8fbe-4397-9dbb-9b2b20218538