[EXPORTER] Remove out of date ETW exporter doc

[ThomsonTan]

Changes

Removed the out of date doc for ETW exporter.

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

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[netlify]

✅ Deploy Preview for opentelemetry-cpp-api-docs canceled.

Name	Link
🔨 Latest commit	75a25c9
🔍 Latest deploy log	https://app.netlify.com/sites/opentelemetry-cpp-api-docs/deploys/67180ae25df5e70008384f5c

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.91%. Comparing base (497eaf4) to head (75a25c9).
Report is 146 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3103      +/-   ##
==========================================
+ Coverage   87.12%   87.91%   +0.79%     
==========================================
  Files         200      195       -5     
  Lines        6109     6133      +24     
==========================================
+ Hits         5322     5391      +69     
+ Misses        787      742      -45     

see 98 files with indirect coverage changes

[marcalff]

Please fix the CI for markdown-lint, will merge after that.