[TEST] Remove YAML Tests

Removes YAML tests for the `/_cluster/allocation/explain` API. The tests
 passed in alternate values for the APIs. An example is passing "true"
 for fields expecting a boolean value. While this is explicitly
 supported by the API, this is not the correct place to be testing this
 behaviour, and resulted in the API specification failing
 validation.

Relates #127028

Author: joshua-adams-1