bootctl: do not fail on removing unfied kernel image

bootctl: do not fail on removing unfied kernel image

A boot loader entry for a unified kernel image has
BootEntry.kernel : path to the image relative to ESP or XBOOTLDR,
BootEntry.path   : path to the image.
Hence, these two effectively point to the same file.

Hence, by unlink command, the image is removed by
```
deref_unlink_file(&known_files, e->kernel, e->root);
```
then later tried again by
```
r = chase_and_unlink(e->path, root, ...);
```
and of course it fails with -ENOENT.

Let's ignore the failure there. We already ignore ENOENT on removal
at various places, especially in deref_unlink_file().

Fixes #38706.
Follow-ups for 8702496bfb0205764569782a9a2ebd11fd80e5e8.