Drop checkout step from release procedure, other cleanups

See #2736

git-svn-id: http://svn.osgeo.org/postgis/trunk@12560 b70326c6-7e19-0410-871a-916f4a2858ee
Automate postgis_download_url setting in doc/postgis.xml

Drop manual step from HOWTO_RELEASE (and change list to unnumbered)
Closes #2735

git-svn-id: http://svn.osgeo.org/postgis/trunk@12562 b70326c6-7e19-0410-871a-916f4a2858ee