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>

setserial.spec

@@ -54,9 +54,6 @@ mkdir -p ${RPM_BUILD_ROOT}/%{_bindir}
 mkdir -p ${RPM_BUILD_ROOT}/%{_mandir}/man8
 make install DESTDIR=${RPM_BUILD_ROOT}
 
-%clean
-rm -rf ${RPM_BUILD_ROOT}
-
 %files
 %defattr(-,root,root,-)
 %doc README rc.serial