Simplify SupersetClient singleton

[kristw]

Is your feature request related to a problem? Please describe.
SupersetClient wrapper creates overhead in maintaining another type and cause confusion as there is the original class and the wrapper.

Describe the solution you'd like
Use same pattern with other singletons, but may involves breaking change to how to get the singleton which affects incubator-superset

Describe alternatives you've considered
Keep it like this

[villebro]

The codebase on this repo has been moved to the main Apache Superset repo, and consequently the repo is in the process of being archived. See the Superset Improvement Proposal for details: apache/superset#13013 . While all currently open issues and PRs will be closed, we encourage you to reopen this issue on the main repo if it is still relevant.