[bt][data] remove L2capSocketChannelRelay and RfcommSocketChannelRelay headers

Now that the SocketChannelRelay template has only one type parameter,
there's no need for header files which define type aliases for the
proper instantiation of the template for L2CAP and RFCOMM. Delete said
header files, accordingly.

Bug: NET-1933
Test: bt-host-unittests (on eve)

Change-Id: I22a591b8846422c007a2c27d095b29e42255a9de
3 files changed