L2CAP: Minor clean up for scheduler and mtu

* After dequeuing a SDU from channel queue end, unregister dequeue,
until that SDU has been sent to link queue end. Since we only support
basic mode for now, we directly convert a SDU to PDU.
* Cleaned up docs.
* Removed unused method in Scheduler interface.
* Removed unnecessary Mtu constants.

Test: run_cert.sh
Bug: 144375926
Change-Id: I50e6d383dac911ccc2010857f6447af9c59a9562