Remove FieldCompiler.mutable (#628)

Author: AdrienVannson

src/betterproto/plugin/models.py

@@ -484,13 +484,6 @@ def repeated(self) -> bool:
     def optional(self) -> bool:
         return self.proto_obj.proto3_optional
 
-    @property
-    def mutable(self) -> bool:
-        """True if the field is a mutable type, otherwise False."""
-        return self.annotation.startswith(
-            ("typing.List[", "typing.Dict[", "dict[", "list[", "Dict[", "List[")
-        )
-
     @property
     def field_type(self) -> str:
         """String representation of proto field type."""