Remove postgis_topology creation from PostGIS install page

[Serhiy]

Why include "CREATE EXTENSION postgis_topology;" in ​https://docs.djangoproject.com/en/dev/ref/contrib/gis/install/postgis/ if it's useless for Django anyway?

[Tim Graham]

Discussion ​here. I don't know if Django might add some features for it in the future, but does the current note cause any harm?

[Serhiy]

Unnecessary action that takes time and causes confusion.
It's so obvious to me that I can't even find the words to explain it. Why would you need a useless action in a tutorial? Adding the note _afterwards_ just causes a "what." reaction.

[Claude Paroz]

What about this:

docs/ref/contrib/gis/install/postgis.txt

@@ -63 +63 @@
     $ createdb  <db name>
     $ psql <db name>
     > CREATE EXTENSION postgis;
-    > CREATE EXTENSION postgis_topology;
 
-No PostGIS topology functionalities are yet available from GeoDjango, so the
-creation of the ``postgis_topology`` extension is entirely optional.
+GeoDjango itself does currently not leverage any of the `PostGIS topology
+functionalities`__, but if you plan to use them at some point you can also
+install the ``postgis_topology`` extension by issuing ``CREATE EXTENSION
+postgis_topology;``.
+
+__ http://postgis.net/docs/Topology.html
 
 .. versionchanged:: 1.8
 

[Serhiy]

Looks great to me.

[Tim Graham]

Suggestion to simplify the wording a bit: "GeoDjango does not currently leverage any PostGIS topology functionality. If you plan to use those features at some point, you can also ..."

[Claude Paroz <claude@…>]

In 6e8cb75e:

Fixed #24672 -- Clarified note about installing postgis_topology

Thanks Serhiy Zahoriya for the suggestion and Tim Graham for the
review.

[Claude Paroz <claude@…>]

In 99bf908:

[1.8.x] Fixed #24672 -- Clarified note about installing postgis_topology

Thanks Serhiy Zahoriya for the suggestion and Tim Graham for the
review.
Backport of 6e8cb75e3e from master.