[ddc] Update the test that declared a custom 'index' in an enum

Landing of https://dart-review.googlesource.com/c/sdk/+/237365 made
the test fail due to a custom declaration of a member named
'index'. This CL adjusts the test, removing the declaration.

Change-Id: I8f967a6ec473cb1aac8690d740d90647e01bc55f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/237584
Auto-Submit: Chloe Stefantsova <cstefantsova@google.com>
Reviewed-by: Mark Zhou <markzipan@google.com>
Commit-Queue: Mark Zhou <markzipan@google.com>

pkg/dev_compiler/test/expression_compiler/expression_compiler_e2e_dart_2_17_test.dart

@@ -216,7 +216,6 @@ void runSharedTests(SetupCompilerOptions setup, TestDriver driver) {
 
       enum E2 with M {
         v1, v2, id_string;
-        int get index => 10;
       }
 
       mixin M on Enum {
@@ -297,7 +296,7 @@ void runSharedTests(SetupCompilerOptions setup, TestDriver driver) {
       await driver.check(
           breakpointId: 'bp',
           expression: 'E2.v2.mixinMethod()',
-          expectedResult: '1000');
+          expectedResult: '100');
     });
   });
 }