Remove useless codes for server_group

Some codes about 'extract_members' from 'create_server_group'
is written in original bp/patch's reviewing
(I650a8f191dea5eab5b4b1828f0b9f65e33edea2a).

But as the design variation, it can't be specified members
in create API (see it in PS29 vs PS30).
But some useless codes are still kept after this feature merged.
So it's necessary to remove them.

Change-Id: I087c8389f2c735a36d656b44c6a3ecfe3a4aa5a3
Closes-Bug: #1325148
Merge "Remove useless codes for server_group"