bta: le_audio: Fix missing group cleanup on test teardown

This fixes missing group cleanup on test case teardown.
The groups that are left from previous test case execution
will affect the succeeding test cases.

Bug: 357472821
Flag: Exempt; trivial change covered with unit tests
Test: atest bluetooth_le_audio_test
Change-Id: Idb5df6b767d4840f5db7f543c748985e0d73aa81