Remove Default Timeout of 1h for Pipeline and Task start

[danielhelfand]

Part of #865

This pull request partially addresses #865. It removes default values for tkn pipeline start and tkn task start for the --timeout option. This will need to eventually also be applied to tkn clustertask start, but there needs to be changes made via #793 before that can take place.

Removing the default should be backwards compatible as pipelines will still apply the 1h default in the event nothing is specified via tkn. This will also respect the default-timeout-minutes property specified in config-defaults.

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Release Notes

Remove 1h default for --timeout for pipeline and task start to allow default-timeout-minutes to be correctly applied

[vdemeester]

/lgtm

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[chmouel]

nice, thanks @danielhelfand 👍