btif: Remove unused synchronization in event_cleanup_stack

Use Case: Repeated BT ON/OFF

Failure: FD leak is observed with ON/OFF stress test
which eventually leads to crash due to unavailability
of FDs.

Steps:
BT ON/OFF

Root Cause: During stack cleanup an unused future
structure is created which is not freed, leading to FD
leak.

Fix: Remove the creation of the future structure

Test: BT ON/OFF
Fixes: 36019324
Change-Id: Id5945da87f00eb7ab77a5f217c95cc43f0136e06