docs: update to reflect removal of DROP_CLIENT_UNKNOWN envvar

Author: SomberNight

docs/environment.rst

@@ -281,14 +281,6 @@ These environment variables are optional:
   version string. For example to drop versions from 1.0 to 1.2 use
   the regex ``1\.[0-2]\.\d+``.
 
-.. envvar:: DROP_CLIENT_UNKNOWN
-
-  Set to anything non-empty to deny serving clients which do not
-  identify themselves first by issuing the server.version method
-  call with a non-empty client identifier. The connection is dropped 
-  on first actual method call. This might help to filter out simple 
-  robots. This behavior is off by default.
-
 
 Resource Usage Limits
 =====================