Comment out the SERVER_STRING variable from our default error documents.

Some people do not like having this information in their error pages, and
it makes sense to not do it by default.  If users want this back, they
can uncomment it.

PR:	9319


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@95692 13f79535-47bb-0310-9956-ffa450edef68