Remove dependency on github.com/janpfeifer/must in ops_generator

[google-labs-jules]

Removed the dependency on github.com/janpfeifer/must from internal/cmd/ops_generator.
Defined local must and must1 helper functions in internal/cmd/ops_generator/main.go using log.Fatalf for error handling.
Updated binary_ops.go and unary_ops.go to use the local helpers instead of the external library.
Removed github.com/janpfeifer/must from go.mod and go.sum.

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PR created automatically by Jules for task 1603601399614147070 started by @janpfeifer

[google-labs-jules]

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