[mlir] DCE friend Dialect::registerDialect (#121728)

Author: makslevental

mlir/include/mlir/IR/Dialect.h

@@ -368,7 +368,6 @@ class Dialect {
   DenseSet<std::pair<TypeID, TypeID>> unresolvedPromisedInterfaces;
 
   friend class DialectRegistry;
-  friend void registerDialect();
   friend class MLIRContext;
 };