[stdlib] Remove _FixedArray16 from GroupInfo.json (#75631)

Author: benrimmington

stdlib/public/core/GroupInfo.json

@@ -122,7 +122,6 @@
         "ContiguousArray.swift",
         "ContiguousArrayBuffer.swift",
         "ContiguouslyStored.swift",
-        "FixedArray.swift",
         "SliceBuffer.swift",
         "SwiftNativeNSArray.swift"],
       "HashedCollections": [