Remove docs sub package from RHEL 6

We see the following error on RHEL 6. We can avoid this error by removing the docs sub package only from RHEL 6.

+ install -pm 644 /home/jenkins/workspace/rh-qpid-dispatch-dist-el6-master/build/BUILD/qpid-dispatch-1.11.0/pre_built/man/man8/qdmanage.8.gz /home/jenkins/workspace/rh-qpid-dispatch-dist-el6-master/build/BUILD/qpid-dispatch-1.11.0/pre_built/man/man8/qdrouterd.8.gz /home/jenkins/workspace/rh-qpid-dispatch-dist-el6-master/build/BUILD/qpid-dispatch-1.11.0/pre_built/man/man8/qdstat.8.gz /home/jenkins/workspace/rh-qpid-dispatch-dist-el6-master/build/BUILDROOT/qpid-dispatch-1.11.0-0.450.4636f359.el6.x86_64/usr/share/man/man8/
+ cp -a '/home/jenkins/workspace/rh-qpid-dispatch-dist-el6-master/build/BUILD/qpid-dispatch-1.11.0/pre_built/doc/qpid-dispatch-1.11.0/*' /home/jenkins/workspace/rh-qpid-dispatch-dist-el6-master/build/BUILDROOT/qpid-dispatch-1.11.0-0.450.4636f359.el6.x86_64/usr/share/doc/qpid-dispatch-1.11.0/
cp: cannot stat `/home/jenkins/workspace/rh-qpid-dispatch-dist-el6-master/build/BUILD/qpid-dispatch-1.11.0/pre_built/doc/qpid-dispatch-1.11.0/*': No such file or directory
error: Bad exit status from /var/tmp/rpm-tmp.53oPfO (%install)

RPM build errors:
Bad exit status from /var/tmp/rpm-tmp.53oPfO (%install)
Traceback (most recent call last):
File "scripts/build", line 20, in <module>
rpm_build(spec_file, "source", "build", build_info)
File "/home/jenkins/workspace/rh-qpid-dispatch-dist-el6-master/scripts/fortworth.py", line 202, in rpm_build
call("rpmbuild -D '_topdir

' -ba

", get_absolute_path(build_dir), spec_file)
File "/home/jenkins/workspace/rh-qpid-dispatch-dist-el6-master/scripts/plano.py", line 720, in call
check_process(proc)
File "/home/jenkins/workspace/rh-qpid-dispatch-dist-el6-master/scripts/plano.py", line 922, in check_process
raise CalledProcessError(proc.returncode, proc.command_string)
subprocess.CalledProcessError: Command 'rpmbuild -D '_topdir /home/jenkins/workspace/rh-qpid-dispatch-dist-el6-master/build' -ba build/SPECS/qpid-dispatch.spec' returned non-zero exit status 1
make: *** [build] Error 1