Remove redundant close method from DedupDb and RethinkDedupDb

I'm trying to implement another DedupDb interface and I looked into the
use of each method. The ``close`` method of ``dedup.DedupDb`` and
``deup.RethinkDedupDb`` is empty.
It is also invoked from ``controller``.

Since it doesn't do anything and it won't in the foreseeable future,
let's remove it.

warcprox/controller.py

@@ -211,8 +211,6 @@ class WarcproxController(object):
 
             if self.proxy.stats_db:
                 self.proxy.stats_db.stop()
-            if any((t.dedup_db for t in self.warc_writer_threads)):
-                self.warc_writer_threads[0].dedup_db.close()
 
             self.proxy_thread.join()
             if self.playback_proxy is not None:

warcprox/dedup.py

@@ -58,9 +58,6 @@ class DedupDb(object):
     def stop(self):
         pass
 
-    def close(self):
-        pass
-
     def sync(self):
         pass
 
@@ -154,9 +151,6 @@ class RethinkDedupDb:
     def stop(self):
         pass
 
-    def close(self):
-        pass
-
     def sync(self):
         pass