L2cap: Remove SecurityPolicy in FixedChannel part

Fixed channel doesn't need security per GAP. Probably some fixed channel
like ATT needs service-level security in its own protocol, but L2CAP
cannot enforce it in L2CAP layer. Probably we can provide some other
security requirements outside L2CAP channel registration.

Test: cert/run --host
Bug: 156054037
Change-Id: I849be7f836378e9e473c31ca3bd3f870dd91ee3f