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.cpp

@@ -26,10 +26,6 @@ void RangeAllocator::initialize_with_range(VirtualAddress base, size_t size)
     m_available_ranges.append({ base, size });
 }
 
-RangeAllocator::~RangeAllocator()
-{
-}
-
 void RangeAllocator::dump() const
 {
     dbgln("RangeAllocator({})", this);