É seguro removê-los manualmente. Também parece que há um relatório de bug arquivado: link
Eu posso relatar o mesmo acontecendo aqui com o kernel 3.13.0-54 e o kernel 3.13.0-55.
-rw-r--r-- 1 root root 1.2M Jun 17 20:03 abi-3.13.0-55-generic
-rw-r--r-- 1 root root 1.2M Jun 19 05:04 abi-3.13.0-57-generic
-rw-r--r-- 1 root root 162K Jun 17 20:03 config-3.13.0-55-generic
-rw-r--r-- 1 root root 162K Jun 19 05:04 config-3.13.0-57-generic
drwxr-xr-x 5 root root 12K Jul 11 15:54 grub
-rw-r--r-- 1 root root 11M Jul 7 21:37 initrd.img-3.13.0-54-generic.old-dkms
-rw-r--r-- 1 root root 27M Jul 4 13:37 initrd.img-3.13.0-55-generic
-rw-r--r-- 1 root root 27M Jul 4 13:37 initrd.img-3.13.0-55-generic.old-dkms
-rw-r--r-- 1 root root 27M Jul 6 18:28 initrd.img-3.13.0-57-generic
-rw-r--r-- 1 root root 173K Mar 12 2014 memtest86+.bin
-rw-r--r-- 1 root root 174K Mar 12 2014 memtest86+.elf
-rw-r--r-- 1 root root 175K Mar 12 2014 memtest86+_multiboot.bin
-rw------- 1 root root 3.3M Jun 17 20:03 System.map-3.13.0-55-generic
-rw------- 1 root root 3.3M Jun 19 05:04 System.map-3.13.0-57-generic
-rw------- 1 root root 5.6M Jun 17 20:03 vmlinuz-3.13.0-55-generic
-rw------- 1 root root 5.6M Jun 19 05:04 vmlinuz-3.13.0-57-generic