rust: alloc: cleanup doctest imports to "kernel vertical" style

rust: alloc: cleanup doctest imports to "kernel vertical" style

Change all imports in the alloc module's doctests to use the "kernel
vertical" import style [1].

While at it, drop imports that are automatically included in doctests.

Link: https://docs.kernel.org/rust/coding-guidelines.html#imports
Reviewed-by: Eliot Courtney <ecourtney@nvidia.com>
Link: https://patch.msgid.link/20260513190946.619810-2-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>