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/939Suggested-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>

Makefile

@@ -1589,7 +1589,7 @@ endif # CONFIG_MODULES
 CLEAN_FILES += include/ksym vmlinux.symvers modules-only.symvers \
 	       modules.builtin modules.builtin.modinfo modules.nsdeps \
 	       compile_commands.json .thinlto-cache rust/test rust/doc \
-	       .vmlinux.objs .vmlinux.export.c
+	       rust-project.json .vmlinux.objs .vmlinux.export.c
 
 # Directories & files removed with 'make mrproper'
 MRPROPER_FILES += include/config include/generated          \