Remove thermostat when getting restrained system in SepTop

[IAlibay]

Was adding tests in ABFEs, and worked out that the way we were getting systems out of ThermodynamicState was causing us to get an AndersenThermostat added to the system.

I'm not sure if this actually had an impact in the end, since multistate sampler does it's own system wrangling on top of a thermodynamic state (I think it might drop the extra thermostat), but this is more prudent.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.41%. Comparing base (e797702) to head (91175ca).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1584      +/-   ##
==========================================
- Coverage   95.45%   93.41%   -2.05%     
==========================================
  Files         174      174              
  Lines       14551    14552       +1     
==========================================
- Hits        13890    13594     -296     
- Misses        661      958     +297     

Flag	Coverage Δ
fast-tests	93.41% <100.00%> (?)
slow-tests	?

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[hannahbaumann]

Thanks @IAlibay for catching this! lgtm!

[github-actions]

No API break detected ✅