Remove AuthenticatorSelection extension

[joegoldman2]

Contributes to #426

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (15263ca) 77.50% compared to head (e30cafc) 77.52%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #435      +/-   ##
==========================================
+ Coverage   77.50%   77.52%   +0.02%     
==========================================
  Files          98       98              
  Lines        2716     2714       -2     
  Branches      445      445              
==========================================
- Hits         2105     2104       -1     
+ Misses        492      491       -1     
  Partials      119      119              

Files	Coverage Δ
...ls/Objects/AuthenticationExtensionsClientInputs.cs	14.28% <ø> (+1.78%)	⬆️
...s/Objects/AuthenticationExtensionsClientOutputs.cs	85.71% <ø> (-1.79%)	⬇️

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[Regenhardt]

The non-extension AuthenticatorSelection is already under test so this looks good to me.

[aseigler]

Good cleanup!