Remove compat spl-x.y.z from the distribution

This was added in 93ce2b4ca5 ("Update
build system and packaging"), which merged the SPL and ZFS trees,
and included in 0.8.0; "the next major release" was 2.0.0

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13316

rpm/redhat/zfs-kmod.spec.in

@@ -89,4 +89,3 @@ rm -rf $RPM_BUILD_ROOT
 
 %files -n kmod-%{kmod_name}-devel
 %{_usrsrc}/%{kmod_name}-%{version}
-%{_usrsrc}/spl-%{version}