Remove the Tracer method from the Span API

[MrAlias]

Using the same tracer that created a span introduces the error where an instrumentation library's Tracer is used by other code instead of their own. There is no requirement from the OpenTelemetry specification to include this method in the Span API and it looks like it was borrowed from OpenTracing, where the instrumentation library concept does not exist.

This removes the Tracer method from the Span interface in the trace package and all methods from implementations.

Resolve #1892
Resolve #1887

[codecov]

Codecov Report

Merging #1900 (7c0059d) into main (ae85fab) will decrease coverage by 0.0%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            main   #1900     +/-   ##
=======================================
- Coverage   79.4%   79.4%   -0.1%     
=======================================
  Files        139     139             
  Lines       7418    7414      -4     
=======================================
- Hits        5894    5889      -5     
- Misses      1278    1279      +1     
  Partials     246     246             

Impacted Files	Coverage Δ
oteltest/harness.go	84.2% <ø> (-0.1%)	⬇️
oteltest/span.go	100.0% <ø> (ø)
sdk/trace/span.go	88.8% <ø> (-0.1%)	⬇️
trace/noop.go	53.8% <ø> (-3.3%)	⬇️
trace/trace.go	86.7% <ø> (ø)
bridge/opentracing/internal/mock.go	72.7% <0.0%> (-1.0%)	⬇️