Change wording of merge request threads counter

What does this MR do?

Directly show count of unresolved threads instead of forcing users to calculate the resolved-resolvable counts in their heads.

The goal is to resolve open threads, not keep a score of how many have been created or resolved.

It removes the now unused resolvedDiscussionsCount() function from app/assets/javascripts/notes/components/discussion_counter.vue.

Screenshots

	Before	After
Has unresolved threads
All threads resolved

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides