Use tableDict for DeltaValue lookbacks

Was abusing the set/getCount machinery before.
diff --git a/Lib/fontTools/ttLib/tables/otBase.py b/Lib/fontTools/ttLib/tables/otBase.py
index 5191ec3..5745fe1 100644
--- a/Lib/fontTools/ttLib/tables/otBase.py
+++ b/Lib/fontTools/ttLib/tables/otBase.py
@@ -574,7 +574,7 @@
 
 			else:
 				table[conv.name] = conv.read(reader, font, table)
-				if conv.isCount or conv.isSize:
+				if conv.isCount:
 					reader.setCount(conv.name, table[conv.name])
 
 		self.postRead(table, font)
@@ -617,8 +617,6 @@
 				writer.writeCountReference(table, conv.name)
 			else:
 				conv.write(writer, font, table, value)
-				if conv.isSize:
-					writer.setCount(conv.name, value)
 	
 	def readFormat(self, reader):
 		pass