Document DefaultSpan, remove DefaultTracer

[lalitb]

Fixes #956 , #949

Changes

• Document use case for DefaultSpan
• Remove DefaultTracer ( we already have NoopTracer and NoopTracerProvider as required by specs )

cc @jsuereth

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

[codecov]

Codecov Report

Merging #959 (22bf451) into main (115a136) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #959   +/-   ##
=======================================
  Coverage   95.36%   95.36%           
=======================================
  Files         160      160           
  Lines        6780     6780           
=======================================
  Hits         6465     6465           
  Misses        315      315           

Impacted Files	Coverage Δ
api/include/opentelemetry/trace/default_span.h	14.29% <ø> (ø)