bluetooth: rfcomm: remove tx meta

Due to the sending buf cannot be referred
by sending layer.
It is unsafe to use `buf` identification
for a transmission, because the buf may
have been newly transmitted when the
sent callback is triggered.

Now instead, when the send completion
callback is received, the upper layer
is notified that a transfer is completed.
If multiple bufs are sent at the same
time, there is no guarantee which buf
is completed when the sent callback
triggered. Therefore, it is recommended
that the caller transfers the next data
block after the previous transfer is
completed.

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