rust: init: remove doctest's `Error::from_errno` workaround

Since commit 5ed14747 ("rust: error: make conversion functions
public"), `Error::from_errno` is public.

Thus remove the workaround added in commit a30e94c2 ("rust: init:
make doctests compilable/testable").

Suggested-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/all/20250526152914.2453949-2-ojeda@kernel.org


Signed-off-by: Benno Lossin <lossin@kernel.org>