Remove extra spaces from MR discussion notes

What does this MR do?

Removes extra spaces from MR discussion notes, which are more effective for users on mobile (or narrow screens).

As positive side effects, .notes_line .notes_line2 class are totally eliminated.

What are the relevant issue numbers?

#46340 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [n/a] Documentation created/updated
• [n/a] Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [n/a] Conforms to the database guides

Closes #46340 (closed)