Eventually update build.py to remove xAvgCharWidth workaround once fonttools is fixed

Once fonttools fixes fonttools/fonttools#2538 then

Line 140 in 139f405

    
           # Irritatingly, pyftmerge does not calculate the average character width correctly. It appears to simply average the xAvgCharWidth value in the two font files. However, given the heavy weighting of wider Japanese characters, we need to recalculate the average and insert it back into the font.

can be updated