Remove another instance of assuming LayerArtist has a zorder callback property

[astrofrog]

Follow-up to #2226

[codecov]

Codecov Report

Merging #2227 (7fdc4c9) into master (8e128fa) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2227      +/-   ##
==========================================
+ Coverage   88.04%   88.07%   +0.03%     
==========================================
  Files         247      247              
  Lines       23164    23162       -2     
==========================================
+ Hits        20394    20401       +7     
+ Misses       2770     2761       -9     

Impacted Files	Coverage Δ
glue/viewers/common/viewer.py	90.68% <100.00%> (-0.08%)	⬇️
glue/app/qt/application.py	79.62% <0.00%> (+1.05%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8e128fa...7fdc4c9. Read the comment docs.