Remove access_test.yml because it is empty and does not validate using PECL YAML extension

Remove access_test.yml because it is empty and does not validate using PECL YAML extension, such as in Acquia Cloud environments.

See https://docs.acquia.com/article/pecl-yaml-serialization-errors.

When using the extension and the script found on that page,

Warning: yaml_parse(): end of stream reached without finding document 0 in Command line code on line 4
./core/modules/system/tests/fixtures/HtaccessTest/access_test.yml is borked
PHP Warning: yaml_parse(): parsing error encountered during parsing: did not find expected key (line 3, column 3), context while parsing a block mapping (line 1, column 1) in Command line code on line 4