cli/command/registry: remove all uses of response message

[thaJeztah]

The message returned by the API is a hardcoded message; the only real information currently returned by the API is whether or not the auth was successul; https://github.com/moby/moby/blob/v2.0.0-beta.0/daemon/server/router/system/system_routes.go#L408-L421

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[codecov-commenter]

Codecov Report

:x: Patch coverage is 65.00000% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cli/command/registry/login.go	65.00%	7 Missing :warning:

:loudspeaker: Thoughts on this report? Let us know!

[thaJeztah]

Flaky test, or did break something?

67.19 === Failed
67.19 === FAIL: cli/command/registry TestLoginWithCredStoreCreds (0.00s)
67.19     login_test.go:80: assertion failed: expected an error, got nil
67.19 
67.19 === FAIL: cli/command/registry TestRunLogin/expired_auth_from_store (0.00s)
67.19     login_test.go:309: assertion failed: expected an error, got nil
67.19 
67.19 === FAIL: cli/command/registry TestRunLogin (0.01s)