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.

Reviewed-by: Finn Behrens <[email protected]>
Tested-by: Benno Lossin <[email protected]>
Tested-by: Andreas Hindborg <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Miguel Ojeda <[email protected]>

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