Kernel: Fix adopt_ref_if_nonnull(new T).release_nonnull() pattern

This does the exact thing as `adopt_ref`, which is a recent addition to
AK.

Note that pointers returned by a bare new (without `nothrow`) are
guaranteed not to return null, so they can safely be converted into
references.

Kernel/Devices/MemoryDevice.cpp

@@ -17,7 +17,7 @@ namespace Kernel {
 
 UNMAP_AFTER_INIT NonnullRefPtr<MemoryDevice> MemoryDevice::must_create()
 {
-    return adopt_ref_if_nonnull(new MemoryDevice).release_nonnull();
+    return adopt_ref(*new MemoryDevice);
 }
 
 UNMAP_AFTER_INIT MemoryDevice::MemoryDevice()