minor #16318 [Messenger] Remove mention to unsupported prefetch_count config option (andrew-demb)

This PR was merged into the 6.0 branch.

Discussion
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[Messenger] Remove mention to unsupported `prefetch_count` config option

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`prefetch_count` was removed in symfony/symfony#41319

Commits
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b8602c4 Remove mention to unsupported `prefetch_count` config option

Author: javiereguiluz

messenger.rst

@@ -1187,7 +1187,6 @@ The transport has a number of options:
 ``password``                                  Password to use to connect to the AMQP service
 ``persistent``                                                                                   ``'false'``
 ``port``                                      Port of the AMQP service
-``prefetch_count``
 ``read_timeout``                              Timeout in for income activity. Note: 0 or
                                               greater seconds. May be fractional.
 ``retry``