[cffLib]: add clear() method to Index class to empty in-place

Lib/fontTools/cffLib/__init__.py

@@ -607,6 +607,9 @@ class Index(object):
 	def getCompiler(self, strings, parent, isCFF2=None):
 		return self.compilerClass(self, strings, parent, isCFF2=isCFF2)
 
+	def clear(self):
+		del self.items[:]
+
 
 class GlobalSubrsIndex(Index):
 

Lib/fontTools/subset/cff.py

@@ -470,9 +470,7 @@ def desubroutinize(self):
 				del pd.Subrs
 			if 'Subrs' in pd.rawDict:
 				del pd.rawDict['Subrs']
-	# clear GlobalSubrsIndex
-	if cff.GlobalSubrs:
-		cff.GlobalSubrs.items = []
+	cff.GlobalSubrs.clear()
 
 
 @_add_method(ttLib.getTableClass('CFF '))