Remove unused public API to_span_ptr

[ThomsonTan]

TC review #946 mentioned to_span_ptr should not be public, remove it as it is not used.

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
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #964 (c154562) into main (d210807) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #964   +/-   ##
=======================================
  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/span.h	100.00% <ø> (ø)