Don't emit framework reference on pack

[clairernovotny]

Fixes #998

[codecov]

Codecov Report

Merging #1007 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1007      +/-   ##
==========================================
- Coverage   96.73%   96.72%   -0.01%     
==========================================
  Files         278      278              
  Lines       38154    38154              
  Branches    10988    10988              
==========================================
- Hits        36907    36904       -3     
- Misses        975      977       +2     
- Partials      272      273       +1

Flag	Coverage Δ
#rxnet	96.72% <ø> (-0.01%)	⬇️

Impacted Files	Coverage Δ
...urce/src/System.Reactive/Disposables/Disposable.cs	93.1% <0%> (-3.45%)	⬇️
.../src/System.Reactive/Linq/Observable/ConcatMany.cs	72.09% <0%> (-2.33%)	⬇️
...ive/Concurrency/ConcurrencyAbstractionLayerImpl.cs	97.33% <0%> (+1.33%)	⬆️

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