stack/btm: Refactor disconnect handler

Make the legacy btm_sco_removed() disconnect handlers return bool
just like the l2c_link_hci_disc_comp() handler does. This will prepare
this code for the ISO Manager that will hook into this legacy code code
as well, but will be based more on a new architecture (similar to GD modules).

Bug: 150670922
Tag: #feature
Sponsor: jpawlowski
Test: compilation
Change-Id: I7a394237c4fd1e5a4b8ddfb9f9801f3e66a263ba