Fix documentation of UI selection

Since change Ie5c658488 it is no longer possible to disable the
PolyGerrit UI. The default UI is PolyGerrit, but can be switched
to GWT by a configuration setting. The documentation was not
updated and still claims that the PolyGerrit UI can be disabled.

Update the documentation to reflect reality.

Change-Id: I98ecbdfa8023f0e439f3f5d5e3613d02a6d0dfcd

Documentation/config-gerrit.txt

@@ -2276,6 +2276,12 @@ NoteDb, Gerrit will not be able to use that instance of NoteDb. The serverId
 used to create the NoteDb will show in the resulting exception message in case
 the value differs.
 
+[[gerrit.ui]]gerrit.ui::
++
+Configure which UI is the default. Valid values are `polygerrit` and `gwt`.
++
+Defaults to `polygerrit`.
+
 [[gitweb]]
 === Section gitweb
 
@@ -5152,11 +5158,6 @@ command.
 
 The format is one Base-64 encoded public key per line.
 
-
-== Configuring the Polygerrit UI
-
-Please see link:dev-polygerrit.html[UI] on configuring the Polygerrit UI.
-
 === Configurable Parameters
 
 site_path::

Documentation/dev-polygerrit.txt

@@ -1,34 +0,0 @@
-= PolyGerrit - GUI
-
-== Configuring
-
-By default both GWT and PolyGerrit UI are available to users.
-
-To make PolyGerrit the default UI but keep GWT as a secondary UI:
-----
-[gerrit]
-        ui = POLYGERRIT
-----
-
-To disable GWT but not PolyGerrit:
-----
-[gerrit]
-        enableGwtUi = false
-        enablePolyGerrit = true
-----
-
-To enable GWT but not PolyGerrit:
-----
-[gerrit]
-        enableGwtUi = true
-        enablePolyGerrit = false
-----
-
-To switch to the PolyGerrit UI you have to add `?polygerrit=1` in the URL.
-
-for example https://gerrit.example.org/?polygerrit=1
-
-To disable PolyGerrit UI, change 1 to 0, which will take you back to GWT UI.
-
-
-More information can be found in the link:https://gerrit.googlesource.com/gerrit/+/master/polygerrit-ui/[README]