Fix toolbox deployments when no backup config is provided

What does this MR do?

In cf1ecb18 we simplified the condition for defining a backup config secret as part of !3699 (merged). However, if a GitLab admin disabled MinIO, the change would require the admin to configure a backup secret unless Google Cloud Storage (GCS) were used as the backend.

Restore the previous behavior of allowing the toolbox to be deployed even if no backups are enabled. We only need to check if that value is set for object storage to be configured. We can drop the check on global.minio.enabled because:

1. For the internal MinIO instance, gitlab.toolbox.backups.objectStorage.config can be left empty.

2. If the internal MinIO instance is disabled, then that means either backups are configured via S3/Azure/GCS or not at all. In the former case, gitlab.toolbox.backups.objectStorage.config must not be empty. If the latter case, then gitlab.toolbox.backups.objectStorage.config can be left blank.

Related issues

Relates to #5536 (closed)

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