Remove those unnecessary statements "return True"

Remove those unnecessary statements "return True"

The reasons to remove them:
* These asynchronous API is not necessary to return a value.
* We do not see the purpose of the statements, because it always
  returns true regardless of success or failure.

These statements are unnecessary and misleading, this commit we will
remove these "return True" (Or just remove "True" if it's not at the
end of a function, in order not to change the execution path).

Affected methods:
* delete_volume
* delete_snapshot
* delete_consistencygroup
* update_consistencygroup
* delete_cgsnapshot

Closes-Bug: #1533997
Change-Id: I38bac64b9eb1be6a4756cfcd3894615d2b849551