rust: alloc: remove `allocator_test`

Given we do not have tests that rely on it anymore, remove
`allocator_test`, which simplifies the complexity of the build.

In particular, it avoids potential issues with `rusttest`, such as the
one fixed at [1], where a public function was added to `Kmalloc` and
used elsewhere, but it was not added to `Cmalloc`; or trivial issues
like a missing import [2] due to not many people testing that target.

The only downside is that we cannot use it in the `macros`' crate
examples anymore, but we did not feel a need for that so far, and anyway
we could support that by running those within the kernel too, which we
may do regardless.

Link: https://lore.kernel.org/rust-for-linux/20250816204215.2719559-1-ojeda@kernel.org/ [1]
Link: https://lore.kernel.org/rust-for-linux/20250816210214.2729269-1-ojeda@kernel.org/

 [2]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/r/20250816211900.2731720-1-ojeda@kernel.org


Signed-off-by: Danilo Krummrich <dakr@kernel.org>