Add __delitem__ method to AsdfFile

[sydduckworth]

Description

• Added __delitem__ magic method to AsdfFile class in _asdf.py
• Expanded test_top_level_tree test in test_api.py to verify that del file[key] works as expected.

Closes #1874

Tasks

• run pre-commit on your machine
• run pytest on your machine
• Does this PR add new features and / or change user-facing code / API? (if not, label with no-changelog-entry-needed)
  • write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see below for change types)
  • update relevant docstrings and / or docs/ page
  • for any new features, add unit tests

news fragment change types...

• changes/<PR#>.feature.rst: new feature
• changes/<PR#>.bugfix.rst: bug fix
• changes/<PR#>.doc.rst: documentation change
• changes/<PR#>.removal.rst: deprecation or removal of public API
• changes/<PR#>.general.rst: infrastructure or miscellaneous change

[braingram]

Thanks! Looks good to me. Feel free to merge at your convenience or let me know if the button isn't green for you.

[zacharyburnett]

neat, I never thought to link to automodapi in the changelog, that's really useful