import: don't assert on missing signature file

import: don't assert on missing signature file

Since ac9edf991142c1597c8d86431ee9bd50c21bce41 even the per-file
verification style uses detached signatures instead of inline ones.
That commit dropped the inline verification fallback, but it didn't
update the raw_pull_job_on_finished()/tar_pull_job_on_finished()
callbacks, which caused the per-file verification to silently fall
through the signature check and eventually hit the now unconditional
assert in pull_verify():

~# importctl pull-tar -mN https://distfiles.gentoo.org/releases/amd64/autobuilds/20260426T153103Z/stage3-amd64-systemd-20260426T153103Z.tar.xz
...
Acquired 1.2G for https://distfiles.gentoo.org/releases/amd64/autobuilds/20260426T153103Z/stage3-amd64-systemd-20260426T153103Z.tar.xz.
Download of https://distfiles.gentoo.org/releases/amd64/autobuilds/20260426T153103Z/stage3-amd64-systemd-20260426T153103Z.tar.xz complete.
SHA256 checksum of https://distfiles.gentoo.org/releases/amd64/autobuilds/20260426T153103Z/stage3-amd64-systemd-20260426T153103Z.tar.xz is valid.
Assertion 'signature_job->state == PULL_JOB_DONE' failed at src/import/pull-common.c:588, function pull_verify(). Aborting.

To fix this, let's drop the now outdated guards from the callbacks,
given that both per-file and per-directory verification uses detached
signatures. Also, drop some dead code along the way, given we no longer
need to even determine the verification style anymore.

With this, the reproducer above now shows a proper error:

...
Acquired 1.2G for https://distfiles.gentoo.org/releases/amd64/autobuilds/20260426T153103Z/stage3-amd64-systemd-20260426T153103Z.tar.xz.
Download of https://distfiles.gentoo.org/releases/amd64/autobuilds/20260426T153103Z/stage3-amd64-systemd-20260426T153103Z.tar.xz complete.
Failed to retrieve signature file, cannot verify. (Try --verify=no?)

Also, both --verify=checksum/--verify=no keep working as expected.

Follow-up for ac9edf991142c1597c8d86431ee9bd50c21bce41.
Resolves: #41895