Revert "[turbofan] add CheckSmi call to String.p.startsWith"

This reverts commit 6d209c9b.

Reason for revert: chromium:941952

Original change's description:
> [turbofan] add CheckSmi call to String.p.startsWith
> 
> Add a CheckSmi call to the value of the position argument to
> String.prototype.startsWith(search, [position]).
> 
> Bug: v8:8400, chromium:939746
> Change-Id: I7462bebe0d3fde605a4c27a34c0d9bb3f0cc1c20
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1514198
> Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
> Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#60216}

TBR=sigurds@chromium.org,bmeurer@chromium.org,usharma1998@gmail.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: v8:8400, chromium:939746
Change-Id: Ifee58b9e57313bbf93bca293e92d88af279a0261
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1524204
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60277}

src/compiler/js-call-reducer.cc

@@ -5191,8 +5191,6 @@ Reduction JSCallReducer::ReduceStringPrototypeStartsWith(Node* node) {
         // Ensure that the {string} is actually a String.
         string = effect = graph()->NewNode(
             simplified()->CheckString(p.feedback()), string, effect, control);
-        position = effect = graph()->NewNode(
-            simplified()->CheckSmi(p.feedback()), position, effect, control);
 
         Node* string_length =
             graph()->NewNode(simplified()->StringLength(), string);

test/mjsunit/regress/regress-939746.js

-// Copyright 2019 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
-
-// String.p.startsWith(x, NaN) shouldn't crash V8 when optimized.
-(function () {
-  function f() { 'a'.startsWith('a', NaN); }
-
-  %PrepareFunctionForOptimization(f);
-  f();
-  f();
-  %OptimizeFunctionOnNextCall(f);
-  f();
-})();
-
-// String.p.startsWith should try to coerce non-numbers to numbers.
-(function() {
-  let wasCalled = false;
-
-  const obj = {
-    [Symbol.toPrimitive]: () => wasCalled = true
-  };
-
-  function f() { ''.startsWith('a', obj); }
-
-  %PrepareFunctionForOptimization(f);
-  f();
-  f();
-  %OptimizeFunctionOnNextCall(f);
-  f();
-
-  assertTrue(wasCalled, "String.p.startsWith didn't attempt to coerce the position argument to a Number.")
-})();