test: remove invalid part of stream2-stderr-sync

One test case in `test-stream2-stderr-sync.js` was creating a TTY
object using an undocumented constructor and passing in fd 2.
However, this is running in a child process and fd 2 is actually
a pipe, not a TTY.

The constructor fails on Windows and causes the handle type to be
left uninitialized, which later causes an assert to fail.

On Unix, the constructor fails to retrieve the windows size but unlike
on Windows, it just leaves the size fields undefined and continues
with initializing the stream type, yielding a semi-usable object.

I could make the Windows version match Unix behavior, but it
seems to me that the test is relying on an implementation detail of
an undocumented API, and the Unix behavior is not necessarily more
correct than the Windows one. Thus it makes more sense to remove this
test.