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>

pngcrush.spec

@@ -35,9 +35,6 @@ docbook2man %{name}.sgml
 %{__install} -D -m0755 %{name} %{buildroot}%{_bindir}/%{name}
 %{__install} -D -m0644 %{name}.1 %{buildroot}%{_mandir}/man1/%{name}.1
 
-%clean
-%{__rm} -rf %{buildroot}
-
 %files
 %defattr(-, root, root, -)
 %doc ChangeLog.html