Fix failing GPU tests (due to incorrect unit for rest_radius)

[zhang-ivy]

Description

This PR removes the rest_radius argument from the PointMutationExecutor in two tests. Instead, we will just use the PointMutationExecutor default rest_radius.

Motivation and context

This PR fixes the failing GPU test_relative.py tests: test_unsampled_endstate_energies_GPU and test_RESTCapableHybridTopologyFactory_energies_GPU. The tests were failing because rest_radius=0.2 was being passed to the PointMutationExecutor, but in #1046, we changed rest_radius to be a unit-bearing quantity, so it really should be rest_radius=0.2 * unit.nanometer.

Resolves #???

How has this been tested?

Change log

Fixes bug in test_unsampled_endstate_energies_GPU and test_RESTCapableHybridTopologyFactory_energies_GPU -- removes unitless rest_radius argument and use default instead. 

[ijpulidos]

Just to note the reason why we need these changes. GPU CI tests were failing with AttributeError here and here, for example.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.36%. Comparing base (0aeb4d4) to head (dba06c0).
Report is 60 commits behind head on main.

Additional details and impacted files

[ijpulidos]

LGTM!