[flang][OpenMP] Fix build break with -Werror

llvm-project/flang/lib/Semantics/resolve-directives.cpp:753:7: error: default label in switch which covers all enumeration values [-Werror,-Wcovered-switch-default]
  753 |       default:
      |       ^
1 error generated.

https://lab.llvm.org/buildbot/#/builders/53/builds/17917

flang/lib/Semantics/resolve-directives.cpp

@@ -750,8 +750,6 @@ public:
       case parser::OmpMapType::Value::Delete:
         ompFlag = Symbol::Flag::OmpMapDelete;
         break;
-      default:
-        break;
       }
     }
     if (!ompFlag) {