Drop stray `breakpoint()` in `RuntimeVars.__setattr__`

Left-over debug trap from the `_runtime_vars` pure get/set
refactor — it fired on *every* struct-form rt-var write (e.g.
via `.update()`), hanging any non-tty / CI / forked actor on
`pdb` stdin.

Surfaced by a `/code-review high` pass on #462.

(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

tractor/runtime/_state.py

@@ -103,7 +103,6 @@ class RuntimeVars(Struct):
         key,
         val,
     ) -> None:
-        breakpoint()
         super().__setattr__(key, val)
 
     def update(