Remove sec_id related code in RFCOMM

That's for BTM security registration.  RFCOMM is not using it.  So far
only L2cap channel (GAP_ConnOpen) is using dynamic sec_id.

Bug: 159815595
Tag: #refactor
Test: compile & verify basic functions working
Change-Id: I4c2bc679d7f311bb240179866839a9ca86df0c00