common: rm the cast operator of md_config_cacher_t

as its replacement - the type-deduced operator() - is now in use,
and the resulting code is more readable and less error-prone.

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>

src/common/config_cacher.h

@@ -50,10 +50,6 @@ public:
     conf.remove_observer(this);
   }
 
-  operator ValueT() const {
-    return value_cache.load();
-  }
-
   ValueT operator*() const {
     return value_cache.load();
   }