rust: delete rust-project.json when running make clean

rust-project.json is the configuration file used by rust-analyzer.
As it is a configuration file and it is not needed to build external
modules, it should be delete by make clean. So, delete rust-project.json
when running make clean.

Link: https://github.com/Rust-for-Linux/linux/issues/939


Suggested-by: Björn Roy Baron <bjorn3_gh@protonmail.com>
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Finn Behrens <fin@nyantec.com>
Acked-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>