Remove the clippy attribute hack in macros

[yannbolliger]

@romac said:

One can register custom attributes on Rust nightly with #![feature(register_tool)]: rust-lang/rust#66070 (would help get rid of the clippy:: hack:

rust-stainless/libstainless_macros/implementation.rs

Lines 183 to 187 in 0b7893d

	/// Note that we simply want to attach some attributes to the item in question. Currently,
	/// Rust doesn't permit user-defined attributes that stay around until the later phases of
	/// rustc. However, any attribute in the `clippy::` group is accepted, and clippy itself
	/// doesn't seem to complain about unknown attributes. We therefore abuse this to attach
	/// some attributes of our own for the stainless extraction pass to detect.

[yannbolliger]

Unfortunately, this doesn't currently work because the feature has to be turned on in the crate. Hence, it is not enough to have

#![feature(register_tool)]
#![register_tool(stainless_lib)]

in the libstainless but it would need to sit in the actual file to verify, which we don't want.