Remove FLAG_min_preparse_length.

test/mjsunit/regress/regress-crbug-659915a.js

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Flags: --allow-natives-syntax --min-preparse-length=10
+// Flags: --allow-natives-syntax
 
 let x;
 function f(a) {
   x += a;
 }
 function g(a) {
   f(a); return x;
 }
 function h(a) {
   x = a; return x;
 }
 
 function boom() { return g(1) }
 
 assertEquals(1, h(1));
 assertEquals(2, boom());
 assertEquals(3, boom());
 %OptimizeFunctionOnNextCall(boom);
 assertEquals(4, boom());