Use default TraceContext propagator in example

[c24t]

This PR updates the flask example to use the W3C propagator following #180.

[toumorokoshi]

There's unit tests that verify b3 propagation. Those should be changed to use TraceContext.

[codecov-io]

Codecov Report

Merging #188 into master will decrease coverage by <.01%.
The diff coverage is 85.97%.

@@            Coverage Diff             @@
##           master     #188      +/-   ##
==========================================
- Coverage   85.32%   85.32%   -0.01%     
==========================================
  Files          38       38              
  Lines        1929     1928       -1     
  Branches      227      227              
==========================================
- Hits         1646     1645       -1     
  Misses        218      218              
  Partials       65       65

Impacted Files	Coverage Δ
opentelemetry-api/src/opentelemetry/util/loader.py	81.25% <ø> (ø)	⬆️
...pentelemetry/context/propagation/httptextformat.py	100% <100%> (ø)	⬆️
...try-ext-wsgi/src/opentelemetry/ext/wsgi/version.py	100% <100%> (ø)	⬆️
...elemetry-api/src/opentelemetry/metrics/__init__.py	88.13% <100%> (+0.2%)	⬆️
opentelemetry-sdk/src/opentelemetry/sdk/util.py	85.88% <100%> (ø)	⬆️
...sts/src/opentelemetry/ext/http_requests/version.py	100% <100%> (ø)	⬆️
...ts/src/opentelemetry/ext/http_requests/__init__.py	89.74% <100%> (ø)	⬆️
...elemetry-api/src/opentelemetry/context/__init__.py	100% <100%> (ø)	⬆️
...him/src/opentelemetry/ext/opentracing_shim/util.py	100% <100%> (ø)	⬆️
...pentelemetry-sdk/src/opentelemetry/sdk/__init__.py	100% <100%> (ø)	⬆️
... and 37 more

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