codecov - fix syntax for excluding code lines from coverage

[pavoljuhas]

The coverage tool understands # pragma: no cover markup comments to exclude
code lines from coverage analysis. The previous syntax # coverage: ignore
had no effect and resulted in spurious coverage changes for a different code
paths taken.

Removing the codecov.yml file, because coverage precision had no effect on
codecov complaints about decreased coverage.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (c193c48) 97.3% compared to head (a4af0e6) 97.3%.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #6240   +/-   ##
======================================
  Coverage    97.3%   97.3%           
======================================
  Files        1116    1116           
  Lines       96174   96154   -20     
======================================
- Hits        93651   93641   -10     
+ Misses       2523    2513   -10     

Files Changed	Coverage Δ
cirq-core/cirq/circuits/circuit_operation_test.py	100.0% <ø> (+0.1%)	⬆️
...ristic_decompositions/two_qubit_gate_tabulation.py	100.0% <ø> (+0.6%)	⬆️
examples/bernstein_vazirani.py	97.0% <ø> (-0.2%)	⬇️
examples/deutsch.py	96.0% <ø> (+2.8%)	⬆️
examples/shor.py	100.0% <ø> (+5.6%)	⬆️

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