Remove cfgs based on nonexisting features

[pmikolajczyk41]

Removing almost 20 cfgs that were using nonexisting features (rust_1_40 and cargo-clippy). They were polluting build output with warnings.

[bjorn3]

rust_1_40 is set in the build script when the rustc version is at least 1.40.

[bjorn3]

cargo-clippy is set when running cargo clippy.

[pmikolajczyk41]

my apologies @bjorn3; in that case, would you allow me to add #![allow(unexpected_cfgs)] instead?

[bjorn3]

Please add appropriate cargo::rustc-check-cfg directives to build.rs instead: https://doc.rust-lang.org/stable/cargo/reference/build-scripts.html#rustc-check-cfg

[pmikolajczyk41]

@bjorn3 for some reason I couldn't make it work with printing cargo::rustc-check-cfg=cfg(feature, values(\"rust_1_40\")) from build.rs (neither with single nor double colon after cargo). It also didn't work when I tried doing it without a feature, just with a raw config rust_1_40. What helped, was adding a lint rule to the Cargo.toml.