Remove deprecated BoringUtils API and Source/Sink annotations.

[dtzSiFive]

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you add appropriate documentation in docs/src?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?
  • As-is, PR title says what I would write there.

Type of Improvement

• API deprecation

Desired Merge Strategy

• Squash: The PR will be squashed and merged (choose this if you have no preference).

Release Notes

Remove deprecated BoringUtils.addSource and BoringUtils.addSink APIs.

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels? (Select the most appropriate one based on the "Type of Improvement")
• Did you mark the proper milestone (Bug fix: 3.6.x, 5.x, or 6.x depending on impact, API modification or big change: 7.0)?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you do one of the following when ready to merge:
  • Squash: You/ the contributor Enable auto-merge (squash) and clean up the commit message.
  • Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.

[dtzSiFive]

Draft until some uses elsewhere are confirmed removed.

Presumably features marked for hard removal are not those there is an expectation of binary compatibility for?
However since that was missed, maybe there still is an expectation and the "real" issue there is that we didn't remove them with 7.0.0, or failing that we didn't update the deprecation messages?

Anyway, just looking to clean things up and inspired by seeing deprecation warnings in our builds.

cc @jackkoenig -- how do you like this handled (these were deprecated in 6.0.0, and marked for removal in 7.0)?