L2cap sec: Remove the workaround for legacy device

We don't enforce security when we need to open an L2cap channel for a
legacy device (pre 2.1, simple pairing not supported), and let upper
layer enforce security.  However, now we always enforce security in
L2cap, except for RFCOMM (which is additional), so there is no other
upper layer can enforce security.  Moreover, supporting pre 2.1 devices
(either local or remote) can give us additional complexity, and GD
doesn't support it.

Bug: 159815595
Tag: #refactor
Test: compile & verify basic functions working
Change-Id: I8bfaa30bb211275aa37122fab991c776a132abcd