Revert "[update-checkout] Clone experimental string processing package"

Author: rxwei

utils/update_checkout/update-checkout-config.json

@@ -77,8 +77,6 @@
             "remote": { "id": "apple/swift-nio" } },
         "swift-nio-ssl": {
             "remote": { "id": "apple/swift-nio-ssl" } },
-        "swift-experimental-string-processing": {
-            "remote": { "id": "apple/swift-experimental-string-processing" } },
         "llvm-project": {
             "remote": { "id": "apple/llvm-project" } }
     },
@@ -122,8 +120,7 @@
                 "swift-markdown": "main",
                 "swift-cmark-gfm": "gfm",
                 "swift-nio": "2.31.2",
-                "swift-nio-ssl": "2.15.0",
-                "swift-experimental-string-processing": "main"
+                "swift-nio-ssl": "2.15.0"
             }
         },
         "rebranch": {