Remove unused multi_adv le adversiting module and its unit test

le_audio is the only client for multi_adv, this module and the unit
tests can be removed once all clients are switched th gd le_advertising_manager

Bug: 290393336
Tag: #refactor
Test: atest --host bluetooth_test_broadcaster bluetooth_test_broadcaster_state_machine  --no-bazel-mode
Change-Id: I1b4a96cd0a00cd401c2a6b906bc6738b6d69437d