Remove comment from pystate created in 2003

[tonybaloney]

This is a very minor change, but in pystate there's a check that thread state shouldn't be null when releasing the GIL.

This is a very useful check. If someone accidentally calls release GIL and didn't acquire the GIL from a native thread in an embedded Python scenario, they get this helpful fatal error message because there is no thread state.

I noticed there is a TODO above this check speculating that it isn't required dating back to 2003.

It is required because it avoids a crash and null lookup.

This PR removes the comment.

[gmarkall]

If this check needs to be kept, does this mean that the following assert(holds_gil(tstate)); is redundant and could be removed too?

[tonybaloney]

If this check needs to be kept, does this mean that the following assert(holds_gil(tstate)); is redundant and could be removed too?

That code is unreachable, so yes