LeAudioService: Clear lead device when removed from the group

When device got unboned, group is destroyed and devices are removed from
the group.
Make sure to clear lead device information if device is removing from
the group.

Bug: 236925456
Test: Manually forget device while streaming
Test: atest BluetoothInstrumentationTests
Tag: #feature
Merged-In: I6fbb8ac3fdf4ad0d1779acd3c72878a645e600ea
Change-Id: I6fbb8ac3fdf4ad0d1779acd3c72878a645e600ea
(cherry picked from commit 77f58a7e)