Remove Linkage::Private

This is the same as Linkage::Internal except that it doesn't emit any
symbol. Some backends may not support it and it isn't all that useful
anyway.

compiler/rustc_monomorphize/src/partitioning.rs

@@ -1240,7 +1240,6 @@ fn collect_and_partition_mono_items(tcx: TyCtxt<'_>, (): ()) -> MonoItemPartitio
                         Linkage::WeakODR => "WeakODR",
                         Linkage::Appending => "Appending",
                         Linkage::Internal => "Internal",
-                        Linkage::Private => "Private",
                         Linkage::ExternalWeak => "ExternalWeak",
                         Linkage::Common => "Common",
                     };