Remove duplicated mapping key in config/locales/en.yml

What does this MR do?

The "errors" section were defined twice in this file, the first one was the once being removed here, another one is at line 164 (before removal) or line 158 (after removal): https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30980/diffs#2e828e23e47c79dd8cad0f080376db81e82d3588_164_158

The content of duplicated strings definition are totally the same, the second part has more data (41 lines in total, compare to the removed 6 lines), so I removed the first part but not the second part.

cc !16710 (merged)

Conformity

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