revert of r808895 somehow missed mod_proxy_ftp

r808895 added `ServerTokens OFF` so some modules were
switched from ap_get_server_banner() to 
ap_get_server_description() so they would never be empty.  

But ap_get_server_description() is documnented as being for local 
use only, so it was not really a suitable replacement much less 
w/ `ServerTokens OFF` subsequently taken out.




git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1851954 13f79535-47bb-0310-9956-ffa450edef68