mbed::fdopen() is provided in mbed_retarget.cpp which will attach a stream to the
given FileHandle. Removing mbed_set_unbuffered_stream() from stream class as it
is defined in mbed_retarget.cpp. Stream class should not decide whether it wants
to detach buffers from c library or not. mbed::fdopen() will do that based upon
isatty() call. So if a FileHandle is not a tty, i.e., is not a device type, c library
buffering will not be turned off. For device type FileHandles, c library buffering
is turned off.