kbuild: remove old Rust docs output path

The Rust code documentation output path moved from `rust/doc` to
`Documentation/output/rust/rustdoc`. The `make cleandocs` target
takes care of cleaning it now since it is integrated with the rest
of the documentation.

Thus remove the old reference.

Fixes: 48fadf4 ("docs: Move rustdoc output, cross-reference it")
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
Link: https://lore.kernel.org/r/20231018160145.1017340-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Makefile

@@ -1474,7 +1474,7 @@ endif # CONFIG_MODULES
 # Directories & files removed with 'make clean'
 CLEAN_FILES += vmlinux.symvers modules-only.symvers \
 	       modules.builtin modules.builtin.modinfo modules.nsdeps \
-	       compile_commands.json .thinlto-cache rust/test rust/doc \
+	       compile_commands.json .thinlto-cache rust/test \
 	       rust-project.json .vmlinux.objs .vmlinux.export.c
 
 # Directories & files removed with 'make mrproper'