remove mutable keyword on LoopOnCpu

[zingale]

we capture by reference, so this doesn't seem required and updates to the AMReX interfaces mean it doesn't work anymore

PR summary

PR motivation

PR checklist

• test suite needs to be run on this PR
• this PR will change answers in the test suite to more than roundoff level
• all newly-added functions have docstrings as per the coding conventions
• the CHANGES file has been updated, if appropriate
• if appropriate, this change is described in the docs

[zingale]

reacting tests pass:
http://groot.astro.sunysb.edu/Castro/test-suite/gfortran/2024-03-19-002/index.html