Remove volume_group conversions from PolicyAidlConversion

Following two methods are defined in both
PolicyAidlConversion.cpp and AidlConversion.cpp

aidl2legacy_int32_t_volume_group_t
legacy2aidl_volume_group_t_int32_t

These are now removed from PolicyAidlConversion

When writing unit tests, audio related libraries are linked
at static libraries and these duplicate defintions throw an
error when linking some tests

Bug: 217292176
Test: Builds

Change-Id: Ie04c7b5734d99eb98af44bb86a27c77d71a3d0bc