Now that the enterprise version of GPDB is only provided via RPM, including
gpseginstall in the distribution would cause conflicts if users try to install
GPDB with RPMs and with gpseginstall on the same cluster.  While it could be
preserved for use by the OSS community, there are several standard tools for
copying GPDB files to segment hosts in a cluster, and recommendations for using
one or more of those tools will be included in the GPDB documentation.

In addition to removing gpseginstall itself, this commit removes references to
it in other utilities' documentation and removes code in gppylib that was only
called by gpseginstall.
Co-authored-by: NJamie McAtamney <jmcatamney@pivotal.io>
Co-authored-by: NKalen Krempely <kkrempely@pivotal.io>
(cherry picked from commit 64014685)