fix(config): align unset help wording with runtime key terminology

[694410194]

Summary

• change the config unset help text to describe removing a configuration key
• keep the runtime success message terminology aligned with the same key wording
• limit the fix to CLI copy only without changing command behavior

Verification

• git diff --check
• static check that ConfigSubcommand::Unset now says configuration key while the success output still says Removed key
• rustc --version (not available in this environment, so cargo-based checks could not be run here)

Fixes #53171
/claim #53171

[greptile-apps]

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