Replace empty destructor bodies with =default definitions

src/plugin/Plugin.cc

@@ -66,8 +66,6 @@ BifItem& BifItem::operator=(const BifItem& other)
 	return *this;
 	}
 
-BifItem::~BifItem() { }
-
 void HookArgument::Describe(ODesc* d) const
 	{
 	switch ( type )

src/plugin/Plugin.h

@@ -190,7 +190,7 @@ private:
 /**
  * A class describing an item defined in \c *.bif file.
  */
-class BifItem
+class BifItem final
 	{
 public:
 	/**
@@ -228,7 +228,7 @@ public:
 	/**
 	 * Destructor.
 	 */
-	~BifItem();
+	~BifItem() = default;
 
 	/**
 	 * Returns the script-level ID as passed into the constructor.