Remove links on NewRoot spans

[MrAlias]

To ensure forwards compatibility, remove the unspecified links currently set on NewRoot spans. In resolving this the parent.GetSpanContextAndLinks was simplified to return a local or remote parent SpanContext only.

Resolves #461

[codecov]

Codecov Report

Merging #1726 (b03b02b) into main (a9b2f85) will increase coverage by 0.1%.
The diff coverage is 80.0%.

@@           Coverage Diff           @@
##            main   #1726     +/-   ##
=======================================
+ Coverage   77.8%   78.0%   +0.1%     
=======================================
  Files        131     131             
  Lines       6984    6955     -29     
=======================================
- Hits        5439    5426     -13     
+ Misses      1295    1279     -16     
  Partials     250     250             

Impacted Files	Coverage Δ
bridge/opentracing/bridge.go	47.8% <0.0%> (+0.1%)	⬆️
internal/trace/parent/parent.go	0.0% <0.0%> (ø)
oteltest/tracer.go	90.6% <ø> (-2.4%)	⬇️
bridge/opentracing/internal/mock.go	73.6% <66.6%> (-0.7%)	⬇️
sdk/trace/span.go	90.4% <100.0%> (ø)
sdk/trace/tracer.go	87.5% <100.0%> (-1.4%)	⬇️