Turn off readahead for RHEL6 distros only

Currently readahead package is excluded for all distros during installation,
actually it's only neccessary to turn off readahead service for RHEL6.
readahead_packages snippet can just be removed.

Bug: 807991
Change-Id: I6d82ca34fb798f1d3b6d069b301b41fc4b70d8f5
Merge "Turn off readahead for RHEL6 distros only" into develop