Revert "[Impeller] fail if software backend is chosen and Impeller is…

… enabled on iOS" (#46217)

Reverts #46124

See b/301660190. This is breaking a few Google iOS tests due to test bed configurations. We'll need more time to investigate fixing those tests before relanding.

shell/platform/darwin/ios/framework/Source/FlutterEnginePlatformViewTest.mm

@@ -72,9 +72,7 @@ - (void)setUp {
                                /*io=*/thread_task_runner);
   platform_view = std::make_unique<flutter::PlatformViewIOS>(
       /*delegate=*/fake_delegate,
-      /*rendering_api=*/fake_delegate.settings_.enable_impeller
-          ? flutter::IOSRenderingAPI::kMetal
-          : flutter::IOSRenderingAPI::kSoftware,
+      /*rendering_api=*/flutter::IOSRenderingAPI::kSoftware,
       /*platform_views_controller=*/nil,
       /*task_runners=*/runners,
       /*worker_task_runner=*/nil,
@@ -92,13 +90,8 @@ - (void)tearDown {
 }
 
 - (void)testMsaaSampleCount {
-  if (fake_delegate.settings_.enable_impeller) {
-    // Default should be 4 for Impeller.
-    XCTAssertEqual(platform_view->GetIosContext()->GetMsaaSampleCount(), MsaaSampleCount::kFour);
-  } else {
-    // Default should be 1 for Skia.
-    XCTAssertEqual(platform_view->GetIosContext()->GetMsaaSampleCount(), MsaaSampleCount::kNone);
-  }
+  // Default should be 1.
+  XCTAssertEqual(platform_view->GetIosContext()->GetMsaaSampleCount(), MsaaSampleCount::kNone);
 
   // Verify the platform view creates a new context with updated msaa_samples.
   // Need to use Metal, since this is ignored for Software/GL.