remove meaningless const qualifiers from plot.h (#2695) #2

Job	Run time
Python 3.10 (omp=n, mpi=n, dagmc=n, ncrystal=n, libmesh=n, event=n vectfit=n)	1h 0m 56s
Python 3.10 (omp=y, mpi=n, dagmc=n, ncrystal=n, libmesh=n, event=n vectfit=n)	2h 25m 0s
Python 3.10 (omp=n, mpi=y, dagmc=n, ncrystal=n, libmesh=n, event=n vectfit=n)	45m 24s
Python 3.10 (omp=y, mpi=y, dagmc=n, ncrystal=n, libmesh=n, event=n vectfit=n)	2h 4m 58s
Python 3.7 (omp=n, mpi=n, dagmc=, ncrystal=, libmesh=, event= vectfit=)	1h 21m 46s
Python 3.8 (omp=n, mpi=n, dagmc=, ncrystal=, libmesh=, event= vectfit=)	1h 11m 30s
Python 3.9 (omp=n, mpi=n, dagmc=, ncrystal=, libmesh=, event= vectfit=)	1h 2m 59s
Python 3.11 (omp=n, mpi=n, dagmc=, ncrystal=, libmesh=, event= vectfit=)	1h 25m 37s
Python 3.10 (omp=y, mpi=y, dagmc=y, ncrystal=, libmesh=, event= vectfit=)	2h 59m 18s
Python 3.10 (omp=n, mpi=n, dagmc=, ncrystal=y, libmesh=, event= vectfit=)	1h 7m 33s
Python 3.10 (omp=y, mpi=y, dagmc=, ncrystal=, libmesh=y, event= vectfit=)	2h 19m 36s
Python 3.10 (omp=y, mpi=n, dagmc=, ncrystal=, libmesh=y, event= vectfit=)	3h 7m 46s
Python 3.10 (omp=y, mpi=n, dagmc=, ncrystal=, libmesh=, event=y vectfit=)	2h 39m 13s
Python 3.10 (omp=n, mpi=y, dagmc=, ncrystal=, libmesh=, event= vectfit=y)	54m 50s
finish	4s
	1d 0h 26m 30s

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