[post] prune extra names already in standard Mac set

This should fix https://github.com/fonttools/fonttools/issues/1119

Running `ftxvalidator -T tt0004c_#1.ttf` no longer produces this error
message, but passes with 'NA'

```
Fatal
    post: The name data overflow the table bounds.
        kATSFontTestSeverityFatalError
```

Lib/fontTools/ttLib/tables/_p_o_s_t.py

@@ -154,7 +154,8 @@ class table__p_o_s_t(DefaultTable.DefaultTable):
 		assert len(glyphOrder) == numGlyphs
 		indices = array.array("H")
 		extraDict = {}
-		extraNames = self.extraNames
+		extraNames = self.extraNames = [
+			n for n in self.extraNames if n not in standardGlyphOrder]
 		for i in range(len(extraNames)):
 			extraDict[extraNames[i]] = i
 		for glyphID in range(numGlyphs):