Remove codecoverage for tests and examples

[godofdream]

Codecov checks Codecoverage for bench and tests folders.
By excluding these files from codecoverage we show more correct codecoverage statistics.

[godofdream]

I removed the "codecov:" line.

[codecov-io]

Codecov Report

Merging #1299 into master will decrease coverage by 2.53%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1299      +/-   ##
==========================================
- Coverage   80.47%   77.93%   -2.54%     
==========================================
  Files         159      146      -13     
  Lines       18541    16980    -1561     
==========================================
- Hits        14921    13234    -1687     
- Misses       3620     3746     +126

Impacted Files	Coverage Δ
tests/test_server.rs	50.65% <0%> (-46.48%)	⬇️
actix-http/src/encoding/decoder.rs	59.82% <0%> (-12.51%)	⬇️
src/test.rs	75.94% <0%> (-8.02%)	⬇️
src/web.rs	84.61% <0%> (-7.7%)	⬇️
awc/src/sender.rs	53.91% <0%> (-6.09%)	⬇️
awc/src/request.rs	75.73% <0%> (-3.31%)	⬇️
actix-http/src/encoding/encoder.rs	69.17% <0%> (-3.01%)	⬇️
actix-http/src/client/h2proto.rs	72.28% <0%> (-2.41%)	⬇️
actix-http/src/response.rs	83.7% <0%> (-0.89%)	⬇️
actix-http/src/h1/encoder.rs	83.58% <0%> (-0.6%)	⬇️
... and 14 more

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[JohnTitor]

Thanks!