Update code coverage integration

[sharwell]

• Simplifies coverage integration
• Removes the need to define CODECOV_TOKEN during builds

[codecov]

Codecov Report

Merging #4543 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4543      +/-   ##
==========================================
+ Coverage   75.11%   75.13%   +0.01%     
==========================================
  Files         908      908              
  Lines      160238   160238              
  Branches    17256    17256              
==========================================
+ Hits       120362   120388      +26     
+ Misses      35060    35033      -27     
- Partials     4816     4817       +1

Flag	Coverage Δ
#Debug	75.13% <ø> (+0.01%)	⬆️
#production	70.52% <ø> (+0.02%)	⬆️
#test	90.31% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
...soft.ML.Data/DataLoadSave/Text/TextLoaderCursor.cs	85.11% <0%> (-0.21%)	⬇️
src/Microsoft.ML.Transforms/Text/LdaTransform.cs	84.6% <0%> (+0.15%)	⬆️
...ML.Transforms/Text/StopWordsRemovingTransformer.cs	86.26% <0%> (+0.15%)	⬆️
src/Microsoft.ML.Maml/MAML.cs	26.21% <0%> (+1.45%)	⬆️
...rosoft.ML.AutoML/ColumnInference/TextFileSample.cs	62.25% <0%> (+2.64%)	⬆️
src/Microsoft.ML.AutoML/Sweepers/Parameters.cs	83.05% <0%> (+5.08%)	⬆️
src/Microsoft.ML.AutoML/Sweepers/ISweeper.cs	67.08% <0%> (+7.59%)	⬆️

[sharwell]

This build appears to have the expected results (no coverage changes). Marking as ready for review.