Remove redundant `clusters_providers_aws.created_by_user_id`
Both Clusters::Cluster
and Clusters::Providers::Aws
models have a user
association.
This is redundant as a provider must belong to a cluster, and so we can retrieve the user record from there instead of storing it twice.
On GitLab.com, clusters_providers_aws.created_by_user_id
is empty for all rows.
Edited by Tiger Watson