stack.cc: remove double deletion of stack_handler_

Calling Clear() is redundant, since it's done by the registry in handle_shut_down.

Test: m com.android.bt
Flag: com::android::bluetooth::flags::same_handler_for_all_modules
Bug: 393449774
Bug: 398195386
Bug: 402005761

Change-Id: I1951393c3fbe7d00940058aca7b361247dea36ad