Remove all unused using statements in the solution

[xiang17]

Removed all unused using statements in the solution. Done with a VS feature.

Changes

Please provide a brief description of the changes here.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Design discussion issue #
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #1354 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1354      +/-   ##
==========================================
+ Coverage   77.63%   77.65%   +0.01%     
==========================================
  Files         226      226              
  Lines        6314     6314              
==========================================
+ Hits         4902     4903       +1     
+ Misses       1412     1411       -1     

Impacted Files	Coverage Δ
...penTelemetry.Exporter.InMemory/InMemoryExporter.cs	87.50% <ø> (ø)
...rc/OpenTelemetry.Exporter.Jaeger/JaegerExporter.cs	84.94% <ø> (ø)
...rc/OpenTelemetry.Exporter.ZPages/ZPagesExporter.cs	90.90% <ø> (ø)
...rc/OpenTelemetry.Exporter.Zipkin/ZipkinExporter.cs	85.89% <ø> (ø)
src/OpenTelemetry/BaseExportProcessor.cs	78.57% <ø> (ø)
src/OpenTelemetry/BaseExporter.cs	64.28% <ø> (ø)
...nTelemetry/Internal/OpenTelemetrySdkEventSource.cs	80.95% <ø> (+3.17%)	⬆️
src/OpenTelemetry/ReentrantExportProcessor.cs	62.50% <ø> (ø)
...emetry.Api/Internal/OpenTelemetryApiEventSource.cs	61.76% <0.00%> (-2.95%)	⬇️
....Prometheus/PrometheusExporterMetricsHttpServer.cs	74.50% <0.00%> (-1.97%)	⬇️
... and 2 more