Kernel: Don't take MM lock in MemoryManager::enter_address_space()

We're not accessing any of the MM members here. Also remove some
redundant code to update CR3, since it calls activate_page_directory()
which does exactly the same thing.

Kernel/Memory/MemoryManager.cpp

@@ -1018,9 +1018,6 @@ void MemoryManager::enter_address_space(AddressSpace& space)
 {
     auto* current_thread = Thread::current();
     VERIFY(current_thread != nullptr);
-    SpinlockLocker lock(s_mm_lock);
-
-    current_thread->regs().cr3 = space.page_directory().cr3();
     activate_page_directory(space.page_directory(), current_thread);
 }