Remove stale `cglobal(:jl_tls_states)` test (#43161)
* Remove stale `cglobal(:jl_tls_states)` test
* Remove `jl_threading_enabled`
* The function `threadingOn()` in `julia-config.jl` is the only reference
other than the tests just removed. It looks like `threadingOn` itself is
not used anywhere in the script.
