Destroy old LSKF-based protector in setLockCredentialWithSpLocked()

Destroying the old LSKF-based protector is a core part of changing the
LSKF, so do it directly in setLockCredentialWithSpLocked() rather than
duplicating it in both callers.  No change in behavior.

Change-Id: Ie4f893f2e9aacf3d3c17dd615929ad6dded8db21