Turn PolyGerrit UI status from experimental to default UI. Remove the
option to disable PolyGerrit UI. This is the next step toward removing
of GWT UI.
We keep gerrit.enableGwtUi option, so that gerrit site administrators can
remove GWT UI and make PolyGerrit the only supported UI already now and
not wait until GWT UI is removed in upcoming releases.
Change-Id: Ie5c658488962911c3a113b08ce1768858f12db07