Remove %clean section

None of currently supported distributions need that.
Last one was EL5 which is EOL for a while.

Signed-off-by: Igor Gnatenko <ignatenkobrain@fedoraproject.org>

rubygem-rdiscount.spec

@@ -109,9 +109,6 @@ cp -a .%{gem_extdir_mri}/{gem.build_complete,*.so} %{buildroot}%{gem_extdir_mri}
 mkdir -p %{buildroot}/%{_bindir}
 mv .%{_bindir}/* %{buildroot}/%{_bindir}
 
-%clean
-rm -rf %{buildroot}
-
 %check
 exit 0
 pushd .%{gem_instdir}