Remove unused var tBTM_SEC_DEV_REC::link_key_changed

set-once-to-default-read-never

Towards readable code

Bug: 163134718
Tag: #refactor
Test: act.py -tc BleCocTest
Test: ble paired 2 phones
Test: classic paired Bose SoundLink
Change-Id: Ife93f8028096744171ae79f2d8608c87e352eff0