Obsolete the TxD/RxD signals

They are not much of use due to the low-level OS limitations as of now, hence
the obsolete proposal. It can be brought back to life at any point when we have
a more clear idea what to do with this API.

"faking" this signal as RealTterm probably does it, is not a nice impression
for the end users. That would be more like a software "signal" (which we
already have in place) rather than a real pinout hardware signal.

It is interesting why Windows, Linux, etc do not provide an option for this.
There must be some strong reason for it.

Change-Id: Ia065e2ee9226d16e724f5e2690b25b954329d78e
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>