slight scanner C code simplification

Cython/Compiler/Scanning.py

@@ -360,10 +360,10 @@ class PyrexScanner(Scanner):
         if sy == IDENT:
             if systring in self.keywords:
                 if systring == u'print' and print_function in self.context.future_directives:
-                    self.keywords.remove('print')
+                    self.keywords.discard('print')
                     systring = EncodedString(systring)
                 elif systring == u'exec' and self.context.language_level >= 3:
-                    self.keywords.remove('exec')
+                    self.keywords.discard('exec')
                     systring = EncodedString(systring)
                 else:
                     sy = systring