chore: fix DCHECK on print job cancellation

[codebytere]

Description of Change

Fixes the following DCHECK:

[25652:0818/224532.220407:FATAL:notification_registrar.cc(52)] Check failed: !IsRegistered(observer, type, source). Duplicate registration.

encountered as a result of callback_.is_null() being checked after std::move was called and so the notification observer not properly being removed.

cc @deepak1556 @zcbenz

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• PR title follows semantic commit guidelines
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.
• This is NOT A BREAKING CHANGE. Breaking changes may not be merged to master until 11-x-y is branched.

Release Notes

Notes: Fixed a potential crash on print job cancellation.

[release-clerk]

Release Notes Persisted

Fixed a potential crash on print job cancellation.

[trop]

I was unable to backport this PR to "10-x-y" cleanly;
you will need to perform this backport manually.

[trop]

@codebytere has manually backported this PR to "10-x-y", please check out #25131