Kernel: Always remove PageDirectories from the cr3 map on destruction

Previously we would only remove them from the map if they were attached
to an AddressSpace, even though we would always add them to the map on
construction. This results in an assertion failure on destruction if
the page directory was never attached to an AddressSpace. (for example,
on an allocation failure of said AddressSpace)

Kernel/Memory/PageDirectory.cpp

@@ -151,8 +151,7 @@ UNMAP_AFTER_INIT void PageDirectory::allocate_kernel_directory()
 PageDirectory::~PageDirectory()
 {
     SpinlockLocker lock(s_mm_lock);
-    if (m_space)
-        cr3_map().remove(cr3());
+    cr3_map().remove(cr3());
 }
 
 }