Fix set_max_idle_connections(0).

This broke during some recent refactorings. The special case branch for
max_connections == 0 wasn't actually setting the max_idle_connections
field.

This change simply deletes that branch, since the existing code that
whittles down the pool size one element at a time suffices. In the
common case this will be set on an empty pool anyhow.

src/pool.rs

@@ -79,13 +79,6 @@ impl ConnectionPool {
             return;
         }
 
-        if max_connections == 0 {
-            // Clear the connection pool, caching is disabled.
-            self.lru.clear();
-            self.recycle.clear();
-            return;
-        }
-
         // Remove any extra connections if the number was decreased.
         while self.lru.len() > max_connections {
             self.remove_oldest();