Make MustDeduplicateAnnotation deletable

[jackkoenig]

Contributor Checklist

• [NA] Did you add Scaladoc to every public function/method?
• [NA] Did you update the FIRRTL spec to include every new feature/behavior?
• 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 state the API impact?
• Did you specify the code generation impact?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• bug fix

API Impact

This expands the API to allow MustDeduplicateAnnotation to be deleted

Backend Code Generation Impact

No impact

Desired Merge Strategy

• Squash

Release Notes

[bugfix] MustDeduplicateAnnotation no longer throws an exception when targets are deleted.

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels?
• Did you mark the proper milestone (1.2.x, 1.3.0, 1.4.0) ?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you mark as Please Merge?