syntax: Remove unused `packed` attribute

[klutzy]

The attribute was removed by #16499.

[rust-highfive]

r? @alexcrichton

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[alexcrichton]

Hm, so technically this is a breaking change as any existing #[packed] attributes will start to be an error instead of being silently ignored, but I believe that this falls under the "easy fix" purview of rust-lang/rfcs#1105, so I think that this is fine to go through.

@bors: r+ d0fc35f

[klutzy]

Yep, I think this is the case of "Minor change: introducing new lint warnings/errors" of the RFC. This patch was made since a Rust beginner misused #[packed] but compiler didn't make a warning.

[bors]

⌛️ Testing commit d0fc35f with merge 8a8583c...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-32-nopt-t, auto-win-32-opt, auto-win-64-nopt-t, auto-win-64-opt

[brson]

As mentioned here this causes stable regressions with crates that were using the #[packed] attribute.