[-Wunsafe-buffer-usage] Remove unused TargetType enum

Author: jkorous-apple

clang/include/clang/Analysis/Analyses/UnsafeBufferUsage.h

@@ -95,8 +95,6 @@ class UnsafeBufferUsageHandler {
 #endif
 
 public:
-  enum class TargetType { Span, Array };
-
   UnsafeBufferUsageHandler() = default;
   virtual ~UnsafeBufferUsageHandler() = default;