remove OpenCV from dependencies

[zacharyburnett]

Resolves JP-3120

This PR removes opencv-python-headless in conjunction with the corresponding effort in stcal (spacetelescope/stcal#138), which produces ellipses from snowballs with respective functionality from scikit-image instead

Checklist for PR authors (skip items if you don't have permissions or they are not applicable)

• added entry in CHANGES.rst within the relevant release section
• updated or added relevant tests
• updated relevant documentation
• added relevant milestone
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below.
  How to run regression tests on a PR
• All comments are resolved
• Make sure the JIRA ticket is resolved properly

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 58.02%. Comparing base (b7e0b10) to head (31db7b0).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8544   +/-   ##
=======================================
  Coverage   58.02%   58.02%           
=======================================
  Files         388      388           
  Lines       38977    38977           
=======================================
  Hits        22617    22617           
  Misses      16360    16360           

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