CacheStorage: Remove stale blink runtime feature.

The CacheStorageAddAllRejectsDuplicates feature was fully launched in
M72 and the backing code was removed in M75.  The runtime feature
entry was missed in the removal of the feature code.

Bug: 720919
Change-Id: Ie6d2fe0d044fe1216fb2724a6c5fdc78f9376529
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1895396Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Ben Kelly <wanderview@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712603}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -266,10 +266,6 @@
     {
       name: "CacheInlineScriptCode"
     },
-    {
-      name: "CacheStorageAddAllRejectsDuplicates",
-      status: "stable",
-    },
     {
       name: "Canvas2dContextLostRestored",
       status: "experimental",