Simplify pip install

The uninstall here has been around since
Ibb4b42119dc2e51577c77bbbbffb110863e5324d.  At the time, there might
have been conflicts between packaged and installed pip.  We don't need
it today; get-pip.py keeps itself separate enough in /usr/local on all
platforms.  Thus we can also remove the suse/centos special-casing.

python3-pip is in the RPM list so we don't need to
re-install for Fedora.

Add a note on why we are over-installing pip.

Remove some old setuptools workarounds that are commented out.

Change-Id: Ie3cb81a8ff71cf4b81e23831c380f83b0381de71
diff --git a/stack.sh b/stack.sh
index 47d6dc7..1aa96c9 100755
--- a/stack.sh
+++ b/stack.sh
@@ -752,7 +752,19 @@
 echo_summary "Installing package prerequisites"
 source $TOP_DIR/tools/install_prereqs.sh
 
-# Configure an appropriate Python environment
+# Configure an appropriate Python environment.
+#
+# NOTE(ianw) 2021-08-11 : We install the latest pip here because pip
+# is very active and changes are not generally reflected in the LTS
+# distros.  This often involves important things like dependency or
+# conflict resolution, and has often been required because the
+# complicated constraints etc. used by openstack have tickled bugs in
+# distro versions of pip.  We want to find these problems as they
+# happen, rather than years later when we try to update our LTS
+# distro.  Whilst it is clear that global installations of upstream
+# pip are less and less common, with virtualenv's being the general
+# approach now; there are a lot of devstack plugins that assume a
+# global install environment.
 if [[ "$OFFLINE" != "True" ]]; then
     PYPI_ALTERNATIVE_URL=${PYPI_ALTERNATIVE_URL:-""} $TOP_DIR/tools/install_pip.sh
 fi