Migrate overrides.js into code

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... before republishing this sim.

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Baseline published before making changes:

• https://phet-dev.colorado.edu/html/natural-selection/1.5.0-dev.2/phet/natural-selection_all_phet.html
• https://phet-dev.colorado.edu/html/natural-selection/1.5.0-dev.2/phet-io/

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This migration has not gone smoothly. I've encountered many problems, including:

Sim-specific design problems:

• https://github.com/phetsims/natural-selection/issues/332
• https://github.com/phetsims/natural-selection/issues/335
• https://github.com/phetsims/natural-selection/issues/336
• https://github.com/phetsims/natural-selection/issues/337

General design problems:

• https://github.com/phetsims/natural-selection/issues/333

Common-code bugs:

• https://github.com/phetsims/axon/issues/437

Futher work on this issue is blocked, because all remaining entries in overrides.js are implicated in one of the above GitHub issues.

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Related issues have been completed, and natural-selection-overrides.js is now empty. Closing.