[devicelab] Remove pool dimension from sub builds.

As the devicelab bots are moving to the try pool, we no longer need to set pool=staging.

The pool dimension will be inherited from the parent build.

Relevant CL: https://crrev.com/i/3240904

Bug: https://github.com/flutter/flutter/issues/53789
Change-Id: Ie60d6c33382ba1df702bddec8ee68c88f8e994c8
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/6184
Reviewed-by: Godofredo Contreras <godofredoc@google.com>
Commit-Queue: Tong Wu <wutong@google.com>