[ttFont] Make setGlyphOrder() clear reverseGlyphOrderMap

Part of https://github.com/fonttools/fonttools/pull/2398

Lib/fontTools/subset/__init__.py

@@ -2864,7 +2864,6 @@ class Subsetter(object):
 				glyphOrder = [g for g in glyphOrder if font.getGlyphID(g) <= self.last_retained_order]
 
 			font.setGlyphOrder(glyphOrder)
-			font._buildReverseGlyphOrderDict()
 
 
 	def _prune_post_subset(self, font):

Lib/fontTools/ttLib/ttFont.py

@@ -413,6 +413,8 @@ class TTFont(object):
 
 	def setGlyphOrder(self, glyphOrder):
 		self.glyphOrder = glyphOrder
+		if hasattr(self, '_reverseGlyphOrderDict'):
+			delattr(self, '_reverseGlyphOrderDict')
 
 	def getGlyphOrder(self):
 		try: