[release-0.24] removing confusing error message

[knative-prow-robot]

This is an automated cherry-pick of #809

🐛 Remove confusing error message `Kafka source is not ready`. This error is returned as an event regardless the Kafka source has any real issue or not and users should not see this error message unless it is a legitimate error or it is an error they can fix.

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Codecov Report

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@@               Coverage Diff               @@
##             release-0.24     #816   +/-   ##
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  Coverage                ?   74.66%           
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  Files                   ?      136           
  Lines                   ?     6181           
  Branches                ?        0           
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  Hits                    ?     4615           
  Misses                  ?     1342           
  Partials                ?      224           

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[steven0711dong]

@googlebot I consent.

[lionelvillard]

/test pull-knative-sandbox-eventing-kafka-unit-tests

[matzew]

/lgtm
/approve

@lionelvillard do we want that on 0.25 too?

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: knative-prow-robot, matzew

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