Zcu: eliminate Decl.alive field

Legacy anon decls now have three uses:
* Type owner decls
* Function owner decls
* `@export` and `@extern`

Therefore, there are no longer any cases where we wish to explicitly
omit legacy anon decls from the binary. This means we can remove the
concept of an "alive" vs "dead" `Decl`, which also allows us to remove
the separate `anon_work_queue` in `Compilation`.

src/codegen.zig

@@ -835,8 +835,6 @@ fn lowerDeclRef(
         return Result.ok;
     }
 
-    try zcu.markDeclAlive(decl);
-
     const vaddr = try lf.getDeclVAddr(decl_index, .{
         .parent_atom_index = reloc_info.parent_atom_index,
         .offset = code.items.len,
@@ -958,8 +956,6 @@ fn genDeclRef(
         }
     }
 
-    try zcu.markDeclAlive(decl);
-
     const decl_namespace = zcu.namespacePtr(decl.src_namespace);
     const single_threaded = decl_namespace.file_scope.mod.single_threaded;
     const is_threadlocal = tv.val.isPtrToThreadLocal(zcu) and !single_threaded;