Merge branch 'remove-feature-flag-errors-view' into 'master'

Remove Feature Flags Errors Partial

See merge request gitlab-org/gitlab!42754

app/views/projects/feature_flags/_errors.html.haml

-#error_explanation
-  .alert.alert-danger
-    - @feature_flag.errors.full_messages.each do |message|
-      %p= message