Remove layouting dead code for non-array SIMD types.

These aren't supported anymore, and are already rejected in type checking.

compiler/rustc_ty_utils/src/errors.rs

@@ -82,12 +82,6 @@ pub(crate) struct ZeroLengthSimdType<'tcx> {
     pub ty: Ty<'tcx>,
 }
 
-#[derive(Diagnostic)]
-#[diag(ty_utils_multiple_array_fields_simd_type)]
-pub(crate) struct MultipleArrayFieldsSimdType<'tcx> {
-    pub ty: Ty<'tcx>,
-}
-
 #[derive(Diagnostic)]
 #[diag(ty_utils_oversized_simd_type)]
 pub(crate) struct OversizedSimdType<'tcx> {