rust: io: remove legacy relaxed accessors of Mmio

rust: io: remove legacy relaxed accessors of Mmio

The relaxed access functionality is now provided by the `RelaxedMmio`
wrapper type, and we don't have any user of the legacy methods left.
Remove them.

Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Acked-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260206-io-v2-4-71dea20a06e6@nvidia.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>