Dropping older CentOS releases as they've been pulled from the mirrors

src/centos.ipxe

@@ -11,15 +11,9 @@ set os CentOS
 set ksdevice eth0
 menu ${os} - ${arch} - Image Sig Checks: [${img_sigs_enabled}]
 item 7.2.1511 ${os} 7.2
-item 7.1.1503 ${os} 7.1
-item 7 ${os} 7.0
 item 6.7 ${os} 6.7
-item other Choose other version [o]
 isset ${osversion} || choose osversion || goto linux_menu
-iseq ${osversion} other || goto centos_skip_read_osversion
 echo ${cls}
-echo -n Enter version: ${} && read osversion
-:centos_skip_read_osversion
 iseq ${osversion} 6.7 && set dir ${menu}/${osversion}/os/${arch} ||
 set dir ${menu}/${osversion}/os/x86_64
 set repo http://${mirror}/${dir}