Refactor `parse_labels()` to remove `return_stripped_text_flag` parameter

[ezraporter]

Currently parse_labels() takes a return_stripped_text_flag argument that, if TRUE, returns a flag indicating whether or not parse_labels() altered label text during parsing by stripping HTML/field embedding logic.

This complexity was introduced in #118 to allow check_parsed_labels() to add a message indicating that the labels were altered. Ideally check_parsed_labels() could produce the correct message without parse_labels() needing to pass a flag around but this would take additional refactoring of how we handle applying labels.

Checklist

• [x] The issue is atomic
• [x] The issue description is documented
• [x] The issue title describes the problem succinctly
• [ ] Developers are assigned to the issue
• [x] Labels are assigned to the issue