Don't try to stop advertising on death of GATT clients

We should no longer stop advertising for GATT clients. Advertisers
are kept in AdvertiseClient, and their death is handled in
AdvertiserDeathRecipient.

Bug: 30622771
Test: Kill app with GATT client registered
Change-Id: I21f246042d1632824ed3d2d8937f13bcafe380ab