make clean should remove temporary .cpp files too

[zingale]

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]

I guess this is kinda redundant though, since it appears that amrex removes all of tmp_build_dir

maybe this is a vestige from when clean and realclean did different things?

[yut23]

Also, CASTRO_AUTO_SOURCE_DIR will only be correct if you specify all the right variables (USE_MPI, DEBUG, etc.) when you call make clean.

[zingale]

right. I think that this goes back to the old difference between make clean and make realclean, which no longer exists anymore