Remove obsolete comments and usage

We do not build and translate docbook XML anymore, so do not need
Generatedocbook anymore. Remove the now obsolete comment and also the
sitepackages usage that was only needed for these.

Change-Id: I383a09b557f67bf490f9bfa4b9e30ebe416d95ac

tox.ini

@@ -80,29 +80,17 @@ commands =
   {toxinidir}/tools/publishdocs.sh publish
 
 [testenv:checklang]
-# Generatedocbook needs xml2po which cannot be installed
-# in the venv. Since it's installed in the system, let's
-# use sitepackages.
-sitepackages=True
 whitelist_externals = doc-tools-check-languages
 commands =
   doc-tools-check-languages doc-tools-check-languages.conf test all
 
 [testenv:buildlang]
 # Run as "tox -e buildlang -- $LANG"
-# Generatedocbook needs xml2po which cannot be installed
-# in the venv. Since it's installed in the system, let's
-# use sitepackages.
-sitepackages=True
 whitelist_externals = doc-tools-check-languages
 commands =
   doc-tools-check-languages doc-tools-check-languages.conf test {posargs}
 
 [testenv:publishlang]
-# Generatedocbook needs xml2po which cannot be installed
-# in the venv. Since it's installed in the system, let's
-# use sitepackages.
-sitepackages=True
 whitelist_externals = doc-tools-check-languages
 commands =
   doc-tools-check-languages doc-tools-check-languages.conf publish all