Delete azure_identity dead code

[chlowell]

Closes #2281

[Copilot]

Pull Request Overview

This PR removes dead code and related dependencies from the azure_identity SDK, cleaning up unused modules and functions.

• Deleted unused modules: refresh_token, oauth2_http_client, chained_token_credential, and authorization_code_flow.
• Removed the dead clear function from the token cache and purged the oauth2 dependency from project configuration.

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
sdk/identity/azure_identity/src/refresh_token.rs	Removed the refresh token functionality which is no longer used.
sdk/identity/azure_identity/src/oauth2_http_client.rs	Deleted the unused oauth2 HTTP client adapter.
sdk/identity/azure_identity/src/lib.rs	Removed module references to the deleted dead code files.
sdk/identity/azure_identity/src/credentials/cache.rs	Removed the unused clear() function from the token cache.
sdk/identity/azure_identity/src/chained_token_credential.rs	Deleted the chained token credential module as it was not in use.
sdk/identity/azure_identity/src/authorization_code_flow.rs	Removed the authorization code flow module that is deprecated.
sdk/identity/azure_identity/Cargo.toml	Removed the oauth2 dependency from the azure_identity Cargo.toml.
eng/dict/crates.txt	Removed oauth2 from the list of crate dependencies.
Cargo.toml	Removed the oauth2 dependency from the root Cargo.toml.

[heaths]

Only one suggestion, but nothing blocking.