[kernel] Remove process & thread self-handles

For the coming switch to cap/handle ref-counting being the main lifetime
determiner of objects, get rid of self handles for threads and processes
to avoid circular references. Instead, passing 0 to syscalls expecting a
thread or process handle signifies "this process/thread".

src/kernel/objects/thread.cpp

@@ -32,8 +32,6 @@ thread::thread(process &parent, uint8_t pri, uintptr_t rsp0) :
         m_tcb.rsp0 = rsp0;
 
     m_creator = current_cpu().thread;
-    m_self_handle = g_cap_table.create(this, thread::parent_caps);
-    parent.add_handle(m_self_handle);
 }
 
 thread::~thread()