pkg/docker/config: Remove GetUserLoggedIn
pkg/docker/config: Remove GetUserLoggedIn

Use GetAuthentication directly, replacing:

  username, err := GetUserLoggedIn(sys, registry)

with:

  username, _, err := GetAuthentication(sys, registry)

This is just as convenient and removes the differences between lookup
logic from the two functions (e.g. GetUserLoggedIn ignored
DockerAuthConfig).  I've removed the function instead of deprecating
at Miloslav's request [1].

GetUserLoggedIn originally landed in f28367e1 (Add docker/config
package to containers/image/pkg, 2017-08-29, #333), and neither the
commit message nor the pull request motivate the diversion from the
GetAuthentication lookup logic.

[1]: https://github.com/containers/image/pull/597#issuecomment-471686182

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

containers/image,containers/image