Kernel: Allow Userspace<T> pointers with invalid content

It's not an error to create a Userspace<T> that points to kernel memory
as the point of Userspace<T> is not to validate the address, but rather
to choose safe overloads that do validation before any data transfer
takes place.

Fixes #4581.

AK/Userspace.h

@@ -60,7 +60,6 @@ public:
     Userspace(FlatPtr ptr)
         : m_ptr(ptr)
     {
-        ASSERT(m_ptr < 0xc0000000);
     }
 
     FlatPtr ptr() const { return m_ptr; }