[Messenger] Remove duplicated Redis transport requirements

The Redis transport requirements has been presented twice. I mean the following sentences: "This transport requires the Redis PHP extension (>=4.3) and a running Redis server (^5.0)." and "To use the Redis transport, you will need the Redis PHP extension (>=4.3) and a running Redis server (^5.0).".

Author: sebpacz

messenger.rst

@@ -1164,9 +1164,6 @@ The Redis transport DSN may looks like this:
 
     The Unix socket DSN was introduced in Symfony 5.1.
 
-To use the Redis transport, you will need the Redis PHP extension (>=4.3) and
-a running Redis server (^5.0).
-
 A number of options can be configured via the DSN or via the ``options`` key
 under the transport in ``messenger.yaml``: