Bluetooth: Remove l2cap_sk_list

A new list was added to replace the socket based one. This new list
doesn't depent on sock and then fits better inside l2cap_core.c code.

It also rename l2cap_chan_alloc() to l2cap_chan_create() and
l2cap_chan_free() to l2cap_chan_destroy)

Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>