Remove outdated coho docs

[janpio]

This PR removes some of the Markdown files in docs that were outdated or replaced with new documents in another repository:

• docs/committer-workflow.md had a fully outdated commit workflow, and the other stuff ("Welcome new committers") was replaced by TODO
• docs/processing-pull-requests.md described a process that is not GitHub native and produces unwanted commits and behavior
• docs/code-reviews.md had no more actual content
• docs/jira-triage.md - we don't use JIRA any more
• docs/storing-repo-versions-design.md contained mostly outdated information but nothing actionable, also replaced via https://github.com/apache/cordova-contribute/pull/24 and https://github.com/apache/cordova-contribute/pull/22
• docs/versioning-and-release-strategy.md was replaced by https://github.com/apache/cordova-contribute/pull/22
• ...