Remove dead code in getindex(::AbstractModel, ::Symbol)

[odow]

We don't use nothing to signify duplicates since we now call _error_if_cannot_register.

And this branch can give a very misleading error if someone actually wants to put nothing in there.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.57%. Comparing base (139bead) to head (28a8f1f).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3868      +/-   ##
==========================================
+ Coverage   97.55%   97.57%   +0.01%     
==========================================
  Files          44       44              
  Lines        6059     6057       -2     
==========================================
- Hits         5911     5910       -1     
+ Misses        148      147       -1     

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