Remove a broken, unused template class

Since Clang changes
llvm/llvm-project#84050 and
llvm/llvm-project#90152 (upcoming in
Clang 19.x), Clang will diagnose member accesses before instantiating
C++ templates.

Within the optional_container_property template, this causes errors
for the calls to this->Copy() and this->clear(), as there are no
corresponding methods within that template class, errors like these:

    asdcplib/src/MXF.h:276:12: error: no member named 'Copy' in 'optional_container_property<PropertyType>'
      276 |             this->Copy(rhs.m_property);
          |             ~~~~  ^
    asdcplib/src/MXF.h:284:48: error: no member named 'clear' in 'optional_container_property<PropertyType>'
      284 |           void reset(const PropertyType& rhs) { this->clear(); }
          |                                                 ~~~~  ^

This template is unused, and these faulty calls have been present
since the class was added in 0291582.

Simply remove the unused template class, to avoid these compiler
errors.

This fixes #136.

src/MXF.h

@@ -263,30 +263,6 @@ namespace ASDCP
 	  const PropertyType& const_get() const { return m_property; }
 	};
 
-      // wrapper object manages optional properties
-      template <class PropertyType>
-	class optional_container_property
-	{
-	  PropertyType m_property;
-
-	public:
-	  optional_container_property() {}
-	optional_container_property(const PropertyType& value) : m_property(value) {}
-	  const optional_container_property<PropertyType>& operator=(const PropertyType& rhs) {
-	    this->Copy(rhs.m_property);
-	    return *this;
-	  }
-
-	  bool operator==(const PropertyType& rhs) const { return this->m_property == rhs; }
-	  bool operator==(const optional_property<PropertyType>& rhs) const { return this->m_property == rhs.m_property; }
-	  operator PropertyType&() { return this->m_property; }
-	  void set(const PropertyType& rhs) { this->m_property = rhs; }
-	  void reset(const PropertyType& rhs) { this->clear(); }
-	  bool empty() const { return ! this->m_property.HasValue(); }
-	  PropertyType& get() { return m_property; }
-	  const PropertyType& const_get() const { return m_property; }
-	};
-
       // base class of all metadata objects
       //
       class InterchangeObject : public ASDCP::KLVPacket