Change response code for successful delete image member to 204.

The delete image member call currently returns 200 with no response
body.  According to the API contract, 200 means to expect a body.
This patch changes the response code to 204.

Change-Id: Id48187b3c47e39075a352d3027dd3d3451d46905
Closes-Bug: #1229366
Merge "Change response code for successful delete image member to 204."