chore(ci): remove completed Doppler import workflow

[P4suta]

Doppler now owns the migrated credentials and its native GitHub Secrets sync has been verified for the existing repository and environment scopes.
Remove the completed one-time import crate and workflow.

Validation: the fixed nonpublishing transfer privately compared source and destination values successfully, and the temporary token was revoked and removed.
Existing environment protections and Immutable Releases remain intact.

[coderabbitai]

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