Delete backspace-for-back RuntimeFeature.

third_party/WebKit/Source/platform/RuntimeEnabledFeatures.json5

 {
   // http://dev.chromium.org/blink/runtime-enabled-features
   //
   // This list is used to generate RuntimeEnabledFeatures.h/cpp which contains
   // a class that stores static enablers for all experimental features.
 
   parameters: {
     // Each feature can be assigned a "status":
     // * status=stable: Enable this in all Blink configurations. We are
     //   committed to these APIs indefinitely.
@@ skipping 94 matching lines @@
     },
     {
       name: "BackgroundSync",
       status: "stable",
     },
     {
       name: "BackgroundVideoTrackOptimization",
       status: "stable",
     },
     {
-      name: "BackspaceDefaultHandler",
-      status: "test",
-    },
-    {
       name: "BlockCredentialedSubresources",
       status: "stable",
     },
     {
       name: "BlockLegacySubresources",
       status: "stable",
     },
     {
       name: "BroadcastChannel",
       status: "stable",
@@ skipping 1005 matching lines @@
     {
       name: "Worklet",
       status: "experimental",
     },
     {
       name: "XSLT",
       status: "stable",
     },
   ],
 }