Disable codecov

[max-sixty]

Currently every PR fails the codecov test, with codecov stats I don't think are correct

Open to a better way to do this, but disabling is better than the current state

[codecov-io]

Codecov Report

Merging #113 into master will decrease coverage by 0.71%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #113      +/-   ##
==========================================
- Coverage   28.49%   27.78%   -0.72%     
==========================================
  Files           4        4              
  Lines        1537     1537              
==========================================
- Hits          438      427      -11     
- Misses       1099     1110      +11

Impacted Files	Coverage Δ
pandas_gbq/tests/test_gbq.py	26.86% <0%> (-1.42%)	⬇️

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[tswast]

We can re-enable as part of #20.