Fix serialization of repository field on Publications

[dralley]

closes #4792
https://pulp.plan.io/issues/4792

Since all publications are have a repository version, there is no need to display the repository field on read ops. We just want to be able to create one from the latest version of a repo.

[codecov]

Codecov Report

Merging #152 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #152      +/-   ##
==========================================
- Coverage    67.4%   67.37%   -0.04%     
==========================================
  Files          64       64              
  Lines        3056     3056              
==========================================
- Hits         2060     2059       -1     
- Misses        996      997       +1

Impacted Files	Coverage Δ
pulpcore/app/serializers/publication.py	92.77% <100%> (ø)	⬆️
pulpcore/app/models/publication.py	77.94% <0%> (-1.48%)	⬇️

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