rust: simplify Clippy warning flags set

All Clippy lint groups that we enable, except `correctness`, have a
default `warn` level, thus they may be removed now that we relaxed all
lints to `warn`.

Moreover, Clippy provides an `all` lint group that covers the groups
we enable by default. Thus just use `all` instead -- the only change is
that, if Clippy introduces a new lint group or splits an existing one,
we will cover that one automatically.

In addition, `let_unit_value` is in `style` since Rust 1.62.0, thus it
does not need to be enabled manually.

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

Author: ojeda

Makefile

@@ -465,10 +465,8 @@ export rust_common_flags := --edition=2021 \
 			    -Wunreachable_pub -Wnon_ascii_idents \
 			    -Wmissing_docs \
 			    -Wrustdoc::missing_crate_level_docs \
-			    -Wclippy::correctness -Wclippy::style \
-			    -Wclippy::suspicious -Wclippy::complexity \
-			    -Wclippy::perf \
-			    -Wclippy::let_unit_value -Wclippy::mut_mut \
+			    -Wclippy::all \
+			    -Wclippy::mut_mut \
 			    -Wclippy::needless_bitwise_bool \
 			    -Wclippy::needless_continue \
 			    -Wclippy::no_mangle_with_rust_abi \