Remove StructGroupSourceReq

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

Bug: 32071916
Test: m
Change-Id: I132190f47cea9f56ec56a824763f7e2b2cfde893

config/compiled-classes-phone

@@ -3824,7 +3824,6 @@ android.system.PacketSocketAddress
 android.system.StructAddrinfo
 android.system.StructFlock
 android.system.StructGroupReq
-android.system.StructGroupSourceReq
 android.system.StructIcmpHdr
 android.system.StructIfaddrs
 android.system.StructLinger

config/preloaded-classes

@@ -2004,7 +2004,6 @@ android.system.PacketSocketAddress
 android.system.StructAddrinfo
 android.system.StructFlock
 android.system.StructGroupReq
-android.system.StructGroupSourceReq
 android.system.StructIfaddrs
 android.system.StructLinger
 android.system.StructPasswd