Remove ACL connection reference counting

In addition to maintaining ACL link control blocks, which carry a
"in_use" flag, ACL links are reference counted in the num_acl links
variable.

The reference counting is thrown off when a SM connection initiated for
pairing is followed up by a GATT connection after pairing completes. The
2nd connection is counted against num_acl, even so other parts of the
code recognize the connection as a duplicate ACL connection to the same
BDA.

This patch removes the unecessary reference counting and relies on the
control block status instead, which is correctly maintained.

Bug: 19019189
Change-Id: I0a5f4d279fc634689d0e8c0ef4ad639d0ce5d687