TST: require unit conversion plugin to remove fallback logic for disp units

[kecnry]

Description

This pull request removes the fallback code for parsing display units from the data viewers and in doing so effectively requires the unit conversion plugin. We should have a discussion about which plugins are considered "mandatory" and how to document that. For now, this PR is just to test to see if there are any failures in the current configs/code logic.

Change log entry

• Is a change log needed? If yes, is it added to CHANGES.rst? If you want to avoid merge conflicts,
  list the proposed change log here for review and add to CHANGES.rst before merge. If no, maintainer
  should add a no-changelog-entry-needed label.

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 milestone set? Set this to bugfix milestone if this is a bug fix and needs to be released ASAP; otherwise, set this to the next major release milestone. Bugfix milestone also needs an accompanying backport label.
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?

[kecnry]

looks like all the failing tests are in Mosviz and Specviz2d (which do not yet have the unit conversion plugin, so is unsurprising). Once #3156 is in, we can bring unit conversion to all configs, and then revisit this 🤞