Redirect cronjob-source to ping-source

[evankanderson]

Fixes #2275

Proposed Changes

• Remove cronjob-source example
• Add an alias to redirect from cronjob-source to ping-source

(I'm still testing that aliases work as I understand from the docs)
/assign @n3wscott

[knative-prow-robot]

@evankanderson: The label(s) /label cherrypick-0.13 cannot be applied. These labels are supported: proposal/0.11, proposal/0.12, proposal/0.13, proposal/0.14, proposal/0.15, proposal/0.16, proposal/0.17, proposal/0.18, proposal/0.19, channel/kafka, channel/natss, source/awssqs, source/camel, source/ceph, source/couchdb, source/github, source/kafka, source/prometheus

In response to this:

Fixes #2275

Proposed Changes

• Remove cronjob-source example
• Add an alias to redirect from cronjob-source to ping-source

(I'm still testing that aliases work as I understand from the docs)
/assign @n3wscott

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[knative-prow-robot]

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This pull-request has been approved by: evankanderson

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[n3wscott]

/lgtm