DevTools: Remove redundant default destructor and forward declarations

Problem:
- Default destructors (and constructors) are in `.cpp` files. This
  prevents the compiler's optimizer from inlining them when it thinks
  inlining is appropriate (unless LTO is used).
- Forward declarations can prevent some optimizations, such as
  inlining of constructors and destructors.

Solution:
- Remove them or set them to `= default` and let the compiler handle
  the generation of them.
- Remove unneeded forward declarations.

Userland/DevTools/UserspaceEmulator/RangeAllocator.h

@@ -14,7 +14,6 @@ namespace UserspaceEmulator {
 class RangeAllocator {
 public:
     RangeAllocator();
-    ~RangeAllocator();
 
     void initialize_with_range(VirtualAddress, size_t);