Revert "nbdkit-plugin-tar should be noarch."

Because the plugins directory is archful (%{_libdir}) it is
not in fact a noarch package.

This reverts commit f192c6da90 and
partially reverts commit 9df870792f.

nbdkit.spec

@@ -331,9 +331,8 @@ This package lets you write Ruby plugins for %{name}.
 %package plugin-tar
 Summary:        Tar archive plugin for %{name}
 License:        BSD
-BuildArch:      noarch
 
-Requires:       %{name} = %{version}-%{release}
+Requires:       %{name}%{?_isa} = %{version}-%{release}
 
 
 %description plugin-tar