Drop commented-out `tractor.pause()` debug hooks

Remove 3 leftover `# await tractor.pause(shield=True)`
/ `# await tractor.pause()` calls in
`maybe_open_context()` that were used during the
`_Cache.run_ctx` teardown race diagnostic session
(PR #436). These are dead commented-out code with no
runtime effect — just noise.

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

tractor/trionics/_mngrs.py

@@ -364,14 +364,11 @@ async def maybe_open_context(
                 f'ctx_key={ctx_key}\n'
                 f'acm_func={acm_func}\n'
             )
-            # await tractor.pause()
             mngr = acm_func(**kwargs)
             resources = _Cache.resources
             entry: tuple|None = resources.get(ctx_key)
             if entry:
                 service_tn, ev = entry
-                # XXX, trace this.
-                # await tractor.pause(shield=True)
                 raise RuntimeError(
                     f'Caching resources ALREADY exist?!\n'
                     f'ctx_key={ctx_key!r}\n'
@@ -460,8 +457,6 @@ async def maybe_open_context(
                 f'acm_func={acm_func}\n'
 
             )
-            # XXX, trace it.
-            # await tractor.pause(shield=True)
 
         if user_registered:
             _Cache.users[ctx_key] -= 1