chore(housekeeping): Normalize how config files filter out unwanted lines

[polarathene]

Description

We have used various ways to filter out unwanted lines (empty, white-space only, comments) from configs when reading them to iterate through. In some cases, we had not filtered for all kinds of unwanted lines, or did not filter at all (even though we probably should have).

This PR has normalized that approach to call a helper method instead (added in #2617), with a common approach to call the filter method applied:

• The changes are mostly converting config file paths into local vars to reference instead. These are staged out into commits for easy review if needed (no commit messages).
• Then normalizes the config file pre-processing step to use the recently added method _get_valid_lines_from_file() (renamed in this PR from earlier _filter_to_valid_lines() due to feedback in #2617 ).
• helpers/relay.sh had a loop that only seems to strip out the unwanted lines for the target config. This doesn't seem necessary and could probably just be a cp instead? I collapsed that loop to call the filtering method and just write the output of that instead.
• helpers/utils.sh:_is_comment() has been removed, as only relay.sh was using it and it no longer seems to serve any purpose.

Relevant history

Looks like initial filtering to avoid issues was introduced via this PR in May 2020.

Type of change

• Improvement (non-breaking change that does improve existing functionality)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• New and existing unit tests pass locally with my changes