Remove Docs for Image Resources

[JeromeJu]

Changes

This commit removes the docs left for the removed Image Resources.

/kind misc

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs included if any changes are user facing
• [n/a] Has Tests included if any functionality added or changed
• Follows the commit message standard
• Meets the Tekton contributor standards (including
  functionality, content, code)
• Has a kind label. You can add one by adding a comment on this PR that contains /kind <type>. Valid types are bug, cleanup, design, documentation, feature, flake, misc, question, tep
• [n/a] Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings)
• [n/a] Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lbernick

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[Yongxuanzhang]

/lgtm

[JeromeJu]

/test pull-tekton-pipeline-alpha-integration-tests