perf: only check for `rustc_trivial_field_reads` attribute on traits, not items, impls, etc.
The checks that are removed in this PR (originally added in #85200) caused a small perf regression: https://github.com/rust-lang/rust/pull/88824#issuecomment-932664761
Since the attribute is currently only applied to traits, I don't think it's worth keeping the additional checks for now.
If/when we decide to apply the attribute somewhere else, we can (partially) revert this and reevaluate the perf impact.
r? `@nikomatsakis` cc `@FabianWolff`
d3e6770efb