Remove obsolete msgmut::or_default()

PiperOrigin-RevId: 633997832
diff --git a/src/google/protobuf/compiler/rust/message.cc b/src/google/protobuf/compiler/rust/message.cc
index 69d6f3f..4c6e8a5 100644
--- a/src/google/protobuf/compiler/rust/message.cc
+++ b/src/google/protobuf/compiler/rust/message.cc
@@ -1000,9 +1000,6 @@
             Self{ inner: $pbr$::MutatorMessageRef::new(_private, msg) }
           }
 
-          #[deprecated = "This .or_default() is a no-op, usages can be safely removed"]
-          pub fn or_default(self) -> Self { self }
-
           fn raw_msg(&self) -> $pbr$::RawMessage {
             self.inner.msg()
           }