MNT: Remove unused AddViewerMessage

[pllim]

Description

AddViewerMessage is not used. Its existence is actually confusing because there is a NewViewerMessage that is actually used and it sounds like the same thing.

Checklist for package maintainer(s)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• Are two approvals required? Branch protection rule does not check for the second approval. If a second approval is not necessary, please apply the trivial label.
• Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• Do the proposed changes follow the STScI Style Guides?
• Are tests added/updated as required? If so, do they follow the STScI Style Guides?
• Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• Did the CI pass? If not, are the failures related?
• Is a change log needed? If yes, is it added to CHANGES.rst?
• Is a milestone set? Milestone is only currently required for PRs related to Imviz MVP.
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?

[codecov]

Codecov Report

Merging #939 (39eded8) into main (43976a1) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #939      +/-   ##
==========================================
+ Coverage   69.38%   69.40%   +0.01%     
==========================================
  Files          69       69              
  Lines        4887     4880       -7     
==========================================
- Hits         3391     3387       -4     
+ Misses       1496     1493       -3     

Impacted Files	Coverage Δ
jdaviz/core/events.py	85.49% <100.00%> (+1.43%)	⬆️

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[havok2063]

Works for me!

[kecnry]

Sure enough, I don't see any other mention in the code, and no good reason to keep it around (we can always add it back later).