Bluetooth: Remove export of l2cap_chan_clear_timer()

The call to l2cap_chan_clear_timer() is not really needed in l2cap_sock.c.
This patch also adds a call to l2cap_chan_clear_timer() to the only place
in __l2cap_sock_close() that wasn't calling it. It's safe call it there
because l2cap_chan_clear_timer() check first for timer_peding().

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