Refactor exporter - step 8

[reyang]

This is a follow up PR of #1078.

Changes

• Removed SimpleActivityProcessor (which is replaced by a synchronous, zero-alloc SimpleExportActivityExporter).
• Removed ActivityExporter (which is replaced by the ActivityExportSync class). Will send another PR to swap the class name (rename ActivityExporterSync to ActivityExporter).

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

• Design discussion issue #
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #1127 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1127      +/-   ##
==========================================
+ Coverage   77.30%   77.33%   +0.02%     
==========================================
  Files         222      220       -2     
  Lines        6284     6256      -28     
==========================================
- Hits         4858     4838      -20     
+ Misses       1426     1418       -8     

Impacted Files	Coverage Δ
...nTelemetry/Internal/OpenTelemetrySdkEventSource.cs	80.95% <ø> (ø)
src/OpenTelemetry/Trace/ActivityProcessor.cs	57.89% <0.00%> (-5.27%)	⬇️
...ter.ZPages/Implementation/ZPagesActivityTracker.cs	100.00% <0.00%> (+2.85%)	⬆️
...ZPages/Implementation/ZPagesExporterEventSource.cs	62.50% <0.00%> (+6.25%)	⬆️

[CodeBlanch]

LGTM