boot - Old kernels not properly removed

My /boot is out of space. Issue is, I've been deleting my old kernels:

$ dpkg -l | grep linux-image
ii  linux-image-4.15.0-36-generic                 4.15.0-36.39                                               amd64        Signed kernel image generic
ii  linux-image-generic                           4.15.0.36.38                                               amd64        Generic Linux kernel image

Looks like 4.15.0-34 and 4.13.0-19 weren't properly deleted:

$ ls -al /boot
total 92545
drwxr-xr-x  5 root root     3072 Oct  6 10:04 .
drwxr-xr-x 26 root root     4096 Oct  3 14:54 ..
-rw-r--r--  1 root root  1537610 Aug 27 10:45 abi-4.15.0-34-generic
-rw-r--r--  1 root root  1537821 Sep 24 10:08 abi-4.15.0-36-generic
-rw-r--r--  1 root root   216905 Aug 27 10:45 config-4.15.0-34-generic
-rw-r--r--  1 root root   216954 Sep 24 10:08 config-4.15.0-36-generic
drwx------  3 root root     4096 Dec 31  1969 efi
drwxr-xr-x  5 root root     1024 Oct 12 08:36 grub
-rw-r--r--  1 root root 15422137 Sep  2 14:30 initrd.img-4.13.0-19-generic
-rw-r--r--  1 root root 58504792 Oct  6 10:04 initrd.img-4.15.0-36-generic
drwx------  2 root root    12288 Jul 29  2015 lost+found
-rw-r--r--  1 root root   182704 Jan 28  2016 memtest86+.bin
-rw-r--r--  1 root root   184380 Jan 28  2016 memtest86+.elf
-rw-r--r--  1 root root   184840 Jan 28  2016 memtest86+_multiboot.bin
-rw-r--r--  1 root root        0 Aug 27 10:45 retpoline-4.15.0-34-generic
-rw-r--r--  1 root root        0 Sep 24 10:08 retpoline-4.15.0-36-generic
-rw-------  1 root root  4044038 Aug 27 10:45 System.map-4.15.0-34-generic
-rw-------  1 root root  4046393 Sep 24 10:08 System.map-4.15.0-36-generic
-rw-------  1 root root  8277752 Sep 24 11:54 vmlinuz-4.15.0-36-generic

apt-get autoremove doesn't help:

$ sudo apt-get autoremove linux-image-4.15.0-34-generic
[sudo] password for matt:
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package 'linux-image-4.15.0-34-generic' is not installed, so not removed
...

No luck with grub either:

$ sudo update-grub
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-4.15.0-36-generic
Found initrd image: /boot/initrd.img-4.15.0-36-generic
Adding boot menu entry for EFI firmware configuration
done

How can I safely (re)delete these? Or do I just rm -f them?