chore(flagd-core): drop redundant disabled.feature and test_disabled.py

v3.8.0 of the test-harness submodule already bundles disabled.feature in evaluator/gherkin/ and the disabled-* flags in evaluator/flags/testkit-flags.json. The hatch build hook auto-copies both into the testkit, so test_evaluator.py picks the disabled scenarios up via get_features_path() without any local files.

Signed-off-by: Todd Baert <todd.baert@dynatrace.com>

Author: toddbaert