Clients no longer write "DirReqStatistics 0" in their saveconf output

Stop modifying the value of our DirReqStatistics torrc option just
because we're not a bridge or relay. This bug was causing Tor
Browser users to write "DirReqStatistics 0" in their torrc files
as if they had chosen to change the config.

Fixes bug 4244; bugfix on 0.2.3.1-alpha.