Drop mirror variable as it's not used anymore

src/boot.cfg

@@ -7,9 +7,6 @@ set site_name netboot.xyz
 # set boot domain
 set boot_domain boot.netboot.xyz
 
-# set mirror domain
-set mirror mirror.rackspace.com
-
 # set location of memdisk
 set memdisk http://${boot_domain}/memdisk