[lldb][ValueObject][NFC] Remove unused SyntheticChildrenFrontEnd member

These `IsValid`/`SetValid` APIs are only ever used from 1 data-formatter
in the Swift LLDB fork. Since all the APIs on
`SyntheticChildrenFrontEnd` are meant to be overriden, there is no good
way to enforce calling `IsValid` from the base. And we should just let
that 1 data-formatter manage its own `IsValid` state.

Author: Michael137

lldb/include/lldb/DataFormatters/TypeSynthetic.h

@@ -28,13 +28,9 @@ class SyntheticChildrenFrontEnd {
 protected:
   ValueObject &m_backend;
 
-  void SetValid(bool valid) { m_valid = valid; }
-
-  bool IsValid() { return m_valid; }
-
 public:
   SyntheticChildrenFrontEnd(ValueObject &backend)
-      : m_backend(backend), m_valid(true) {}
+      : m_backend(backend) {}
 
   virtual ~SyntheticChildrenFrontEnd() = default;
 
@@ -100,7 +96,6 @@ class SyntheticChildrenFrontEnd {
                                                 CompilerType type);
 
 private:
-  bool m_valid;
   SyntheticChildrenFrontEnd(const SyntheticChildrenFrontEnd &) = delete;
   const SyntheticChildrenFrontEnd &
   operator=(const SyntheticChildrenFrontEnd &) = delete;