Remove the Non-ThreadSafe Sbt.i

[andrews-unity]

I am not sure this is really needed and just allows users to do bad things, I understand the convenience but I think it just allows badness.

If its going to stick around some runtime checking should be added to make sure its never being used on a different thread.

[wotakuro]

Not to allocate memory every operation , I have to put "Sbt" Object on static field.
So I implements this interface.

But as you said , I don't expect that users do bad things.

I changed the code.Before access to Sbt.i , thread check logic runs.