Remove redundant check

[mejrs]

We still check for rustc_on_unimplemented on implementations, but this functionality was removed in #139091, since then it always returns Ok when called with a non-trait defid.

rust/compiler/rustc_trait_selection/src/error_reporting/traits/on_unimplemented.rs

Lines 557 to 564 in b4c8b0c

	pub fn of_item(tcx: TyCtxt<'tcx>, item_def_id: DefId) -> Result<Option<Self>, ErrorGuaranteed> {
	if !tcx.is_trait(item_def_id) {
	// It could be a trait_alias (`trait MyTrait = SomeOtherTrait`)
	// or an implementation (`impl MyTrait for Foo {}`)
	//
	// We don't support those.
	return Ok(None);
	}

[rustbot]

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[compiler-errors]

r? compiler-errors @bors r+ rollup

[bors]

📌 Commit 1ec3e62 has been approved by compiler-errors

It is now in the queue for this repository.