rust: rbtree: simplify finding `current` in `remove_current`

rust: rbtree: simplify finding `current` in `remove_current`

The previous version used a verbose `match` to get
`current`, which may be slightly confusing at first
glance.

This change makes it shorter and more clearly expresses
the intent: prefer `next` if available, otherwise fall
back to `prev`.

Signed-off-by: Onur Özkan <work@onurozkan.dev>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://lore.kernel.org/r/20250708075850.25789-1-work@onurozkan.dev
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>