Fix Session delete to return 200

the tests on
https://gerrit.openbmc-project.xyz/c/openbmc/openbmc/+/43984

 identified
that there are some that actually look at response codes, and expect
200.  This would show up as failures that looked like:

Test SSL Connection :: This testcase is for testing the SSL connec... | [31mFAIL[0m |
Parent suite setup failed:
ValueError: The HTTP status code was not valid:
status:                                           204
valid_status_codes:
  [0]:                                            200


This commit fixes the behavior change to move back to the old behavior,
and causes SessionService Delete to return 200 with a success message.
This commit changes this code back to 200, even though 204 is valid and
the test should pass for both, 200 with a success message more closely
follows Redfish.

Tested:
Code builds.  Expect the above bump to test this behavior directly.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: I1d6bbfab867dc757c78f745119cfd9835ebbc505