Simplify btm_remove_acl

If btm_sec already sent HCI disconnect, and we send it again, we get bad
handle error, but it's ok.  Other users are sending disconnect without
checking btm_sec.

Bug: 159815595
Tag: #refactor
Test: compile & verify basic functions working
Change-Id: Ib232ff07b2080d574e2516fcdac3b3d6b980bd72