Remove REST server's secure mode altogether

[alessio]

It shouldn't be gaiacli REST server's responsibility to deal in secure layer's business.
We drop secure mode altogether.

As agreed with @zmanian.

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.
• Wrote tests
• Updated relevant documentation (docs/)
• Added a relevant changelog entry: sdkch add [section] [stanza] [message]
• rereviewed Files changed in the github PR explorer

---

For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #3938 into develop will increase coverage by 0.1%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           develop    #3938     +/-   ##
==========================================
+ Coverage    60.22%   60.32%   +0.1%     
==========================================
  Files          197      196      -1     
  Lines        14628    14488    -140     
==========================================
- Hits          8809     8740     -69     
+ Misses        5225     5167     -58     
+ Partials       594      581     -13

[cwgoes]

ACK, looks fine to me.

[sabau]

Let's leave this effort to nginx (or any other proxy) 👍

[jackzampolin]

🗡+5,-349 🔪 LGTM! This is the type of PR I like!