main/shim/acl: simplify LE connection shim

* get rid of LinkConnectionInterface - it was never used
* ACL_AcceptLeConnectionFrom now returns void. Value it returned was
  always true.
* acl_api would call directly to GD. Right now it just post call to GD
  on stack thread. First thing GD connection manager do is post task to
  stack thread, so we are safe removing it.

Test: atest net_test_conn_multiplexing
Bug: 372202918
Flag: EXEMPT, redundant code deletion
Change-Id: Ibcd5c586fae776eeeb9b96c9e91905adb263c164