remove unnecessary import

Author: zhichen3

Exec/science/xrb_spherical/analysis/slice.py

@@ -10,7 +10,6 @@
 import matplotlib.pyplot as plt
 from mpl_toolkits.axes_grid1 import ImageGrid
 from yt.frontends.boxlib.api import CastroDataset
-from yt.units import cm
 
 
 def slice(fnames:List[str], fields:List[str],