Remove unused lint exceptions.

[lemmih]

Summary of changes
Changes introduced in this pull request:

• Remove lint exceptions for absurd_extreme_comparisons, suspicious_op_assign_impl, unreachable_code, unused_variables, and clippy::all.

Reference issue to close (if applicable)

The remaining lint exceptions are related to style rather than correctness (eg. clippy::too_many_arguments and clippy::type_complexity).

Other information and links

[elmattic]

Just a question on the removal of those lint exceptions. If I understand correctly those exceptions were out of date?
It doesn't have been disable because of some removal of clippy lints?

I would rather have all possible checks and lints done, for exemple those kind of code should raise a red flag (suspicious_op_assign_impl):

impl AddAssign for Foo {
    fn add_assign(&mut self, other: Foo) {
        *self = *self - other;
    }
}

[lemmih]

Yes, removing the exceptions means more lints are checked. The suspicious_op_assign_impl lint is enabled by default but was manually suppressed. Fortunately none of the correctness-related lint exceptions are required anymore and could just be removed.

[lemmih]

Why does it believe I haven't signed the CLA?