rust: alloc: remove our fork of the `alloc` crate

It is not used anymore as `VecExt` now provides the functionality we
depend on.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
Link: https://lore.kernel.org/r/20240328013603.206764-5-wedsonaf@gmail.com


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>