GroovyPostbuildRecorder.java: add a way to removeBadgesOnly()

[jimklimov]

As discovered during investigation that led to jenkinsci/badge-plugin#362 this plugin's removeBadges() action actually removes both badges and summaries (is not limited to specifically BadgeAction).

There are a couple of ways out of this:

• Define a new method name for the new activity = least surprise for existing consumers that might rely on this (mis-)behavior.
• Accept that this was a bug and redefine removeBadges() to remove only badges = least surprise for consumers of the current Badge API plugin, and more intuitively matching the name.
  • If anyone wanted a way to remove any derivative of AbstractBadgeAction in one go, a separate method specifically for that can be used, again more intuitively fitting; maybe add a namesake to Badge plugin too for easiest co-existence. This would be effectively about swapping the names and updating javadocs of methods impacted by this PR's initial commit.
  • The Badge plugin currently does remove only badges and only summaries in respective methods, so going with this option is arguably least-surprise as well (ecosystem wide coherence as well as intuitively matching the name to activity).

For now, I went with the first option, but consider the second one superior (albeit a slightly breaking change, potentially). Up to maintainers :)

Testing done

None yet, a theoretical fix.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
  • Subject to maybe going with the second option described above and using different names in the end
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed