trace: remove unused Tracer::invalid method

[jtescher]

This method is not part of the trace spec and is not currently being used in a meaningful way.

[codecov]

Codecov Report

Merging #683 (72de1f9) into main (457556b) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #683      +/-   ##
==========================================
+ Coverage   63.76%   63.85%   +0.09%     
==========================================
  Files          95       95              
  Lines        7733     7717      -16     
==========================================
- Hits         4931     4928       -3     
+ Misses       2802     2789      -13     

Impacted Files	Coverage Δ
opentelemetry/src/global/trace.rs	22.95% <ø> (+0.72%)	⬆️
opentelemetry/src/sdk/trace/tracer.rs	91.00% <ø> (+3.07%)	⬆️
opentelemetry/src/trace/tracer.rs	67.50% <ø> (ø)
opentelemetry/src/trace/noop.rs	75.25% <100.00%> (-0.75%)	⬇️

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