Remove unused btm_le_on_advertising_set_terminated

This event is handled in LeAdvertisingManager, and there
are no observers registered, so it would be a no-op either
way.

Bug: 301661850
Test: mma -j32
Flag: EXEMPT, dead code removal
Change-Id: Ie74f69075928561c39d8729440c43c7bec5251d9