IndexedDB: Remove RuntimeEnabledFlag IDBTransactionCommit.

IDBTransactionCommit has been "stable" since M76. At this point, it is a
permanent part of the Web platform.

Bug: 898257
Change-Id: I8fbb5574c01a5f36dc1d785451f5f4cf7b21510c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1806163
Auto-Submit: Victor Costan <pwnall@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696735}

third_party/blink/renderer/modules/indexeddb/idb_transaction.idl

@@ -47,10 +47,7 @@ enum IDBTransactionMode {
 
     // Methods
     [RaisesException] IDBObjectStore objectStore(DOMString name);
-    [
-      RaisesException,
-      RuntimeEnabled=IDBTransactionCommit
-    ] void commit();
+    [RaisesException] void commit();
     [RaisesException] void abort();
 
     // Events

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -779,10 +779,6 @@
       name: "IDBRelaxedDurability",
       status: "experimental",
     },
-    {
-      name: "IDBTransactionCommit",
-      status: "stable",
-    },
     {
       name: "IdleDetection",
       status: "experimental",