Remove dummy from cirq-core

[dstrain115]

• Latest best practice advice is to remove all mentions of the word 'dummy' as it is problematic terminology.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (26dbabc) 97.84% compared to head (69380e2) 97.84%. Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #6356 +/- ## ========================================== - Coverage 97.84% 97.84% -0.01% ========================================== Files 1110 1110 Lines 96696 96696 ========================================== - Hits 94612 94608 -4 - Misses 2084 2088 +4 ```

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[dstrain115]

@pavoljuhas Added a bunch more references, PTAL, thanks!