Remove BTA layer components for LE advertising

Advertising is so simple, it doesn't need the BTA layer.
Values returned by BTM_VendorSpecificCommand shouldn't be handled as
success or failure, it's just to inform wether we sent the callback.

Bug: 30622771
Change-Id: I104c4fbf84d3818b76fe8c527fbde4e6c4336c3a