Bluetooth: Classic: RFCOMM: Remove TX thread from DLC

There are two main issues found with using DLC TX thread,
Issue 1, the RAM consumption. Every DLC will have a dedicated thread
and thread stack.
Issue 2, the thread stack overflow issue. There is no way to strike a
balance between stack size and RAM consumption. Since the deep of call
stack is depended on the upper layer, the thread stack needs to set by
application. Due to the thread stack of DLC is dedicated, RAM
consumption is the product of the added value and the number of DLCs.

Use a TX worker to replace DLC TX thread.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>