Remove proposed api attribute

[JoeRobich]

Fixes #3872

We aren't using any proposed apis.

[codecov]

Codecov Report

Merging #4029 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4029   +/-   ##
=======================================
  Coverage   85.99%   85.99%           
=======================================
  Files          60       60           
  Lines        1856     1856           
  Branches      214      214           
=======================================
  Hits         1596     1596           
  Misses        200      200           
  Partials       60       60           

Flag	Coverage Δ
#integration	100.00% <ø> (ø)
#unit	85.99% <ø> (ø)

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[JoeRobich]

Passing CI https://travis-ci.org/github/OmniSharp/omnisharp-vscode/builds/722933765