Remove StructGroupSourceReq

- Remove the class as part of removing setsockoptGroupSourceReq from
Os.java
- CtsLibcoreTestCases and CtsLibcoreOjTestCases pass on aosp/master

Bug: 32071916
Test: cts-tradefed run cts-dev --module CtsLibcoreTestCases
Test: cts-tradefed run cts-dev --module CtsLibcoreOjTestCases
Merged-In: I132190f47cea9f56ec56a824763f7e2b2cfde893
Change-Id: I132190f47cea9f56ec56a824763f7e2b2cfde893