remove non-working reference file tests

[braingram]

Description

The tests in asdf._tests.test_reference_files.py have not been running since Feb 2022 (#1079) since they looked for asdf-standard as it existed when it was a submodule:

asdf/asdf/_tests/test_reference_files.py

Line 11 in 867c280

_REFFILE_PATH = os.path.join(os.path.dirname(__file__), "..", "..", "asdf-standard", "reference_files")

This PR removes the tests and asdf-format/asdf-standard#440 adds them to asdf-standard (where the reference files are available).

Fixes: #1522

Checklist:

• pre-commit checks ran successfully
• tests ran successfully
• for a public change, a changelog entry was added
• for a public change, documentation was updated
• for any new features, unit tests were added

[zacharyburnett]

I was just looking at these tests yesterday and wondering how they worked