rust: remove unneeded `kernel::prelude` imports from doctests

Rust doctests implicitly include `kernel::prelude::*`.

Removes explicit `kernel::prelude` imports from doctests.

Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://github.com/Rust-for-Linux/linux/issues/1064


Signed-off-by: Nell Shamrell-Harrington <nells@linux.microsoft.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20240411225331.274662-1-nells@linux.microsoft.com


[ Add it back for `module_phy_driver`'s example since it is within a `mod`,
  and thus it cannot be removed. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>