upstream: remove keepalive close event on cleanup

[PettitWesley]

Resolves #6821

Currently the keepalive dropped connection event is only removed in the async case. However, sync connections can also have keepalive. This PR ensures the event is always removed if it was added.

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Testing
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[PettitWesley]

@edsiper @leonardo-albertovich Can we get this bug fix reviewed and merged? IIRC we had a meeting in February and Eduardo said that Leonardo would review it.

[edsiper]

cc: @leonardo-albertovich

[PettitWesley]

@leonardo-albertovich tested it by adding a log statement as we discussed:

an async IO plugin: cloudwatch_logs:

[2023/04/11 23:25:06] [debug] [aws_credentials] Requesting credentials from the EC2 provider..
[2023/04/11 23:25:06] [error] [KA DROPPED] added
[2023/04/11 23:25:06] [debug] [upstream] KA connection #43 to logs.us-west-2.amazonaws.com:443 is now available
[2023/04/11 23:25:06] [debug] [output:cloudwatch_logs:cloudwatch_logs.0] PutLogEvents http status=200
[2023/04/11 23:25:06] [debug] [output:cloudwatch_logs:cloudwatch_logs.0] Sent events to test-dummy
[2023/04/11 23:25:06] [debug] [output:cloudwatch_logs:cloudwatch_logs.0] Sent 9 events to CloudWatch
[2023/04/11 23:25:06] [debug] [out flush] cb_destroy coro_id=0
[2023/04/11 23:25:06] [debug] [task] destroy task=0x7fc3981a67b0 (task_id=0)
[2023/04/11 23:25:07] [debug] [input chunk] update output instances with new chunk size diff=36
[2023/04/11 23:25:14] [debug] [upstream] KA connection #43 to logs.us-west-2.amazonaws.com:443 has been disconnected by the remote service
[2023/04/11 23:25:14] [error] [KA DROPPED] removed

and a (currently in master) sync IO plugin: s3:

[2023/04/11 23:30:11] [error] [KA DROPPED] added
[2023/04/11 23:30:11] [debug] [upstream] KA connection #21 to s3.us-east-1.amazonaws.com:443 is now available
[2023/04/11 23:30:11] [debug] [output:s3:s3.0] PutObject http status=200
[2023/04/11 23:30:11] [ info] [output:s3:s3.0] Successfully uploaded object /fluent-bit-logs/8967236f1ca7/2023/04/11/23/30/01-objectz36fXgLT
[2023/04/11 23:30:11] [error] [KA DROPPED] removed