LeAudioService: Remove reduntant mActiveDeviceGroupId variable

Information about active group is already in the mGroupDescription.
If needed it can be retrieved from there.
Additional variable requires additional syncing.

Bug: 150670922
Tag: #feature
Sponsor: jpawlowski@
test: Test: atest BluetoothInstrumentationTests
Change-Id: I4105ac0dab88be698c5ffdf70dd759a3d4cc1123