Remove checking of non empty resp body for 204

In case API respond with 204 status code which mean empty response
body, test case does not need to explicitly check the empty response
body. General check for the same is done in rest_client'
response_checker()

This patch removes those checks of empty response body in tests.

Change-Id: I34537968c1a58e6e73022637a71a3a8091ac214c
Merge "Remove checking of non empty resp body for 204"
Merge branch 'master' of https://github.com/openstack/tempest