Remove ensureHeaderWritten() from writeNode()

Remove ensureHeaderWritten() from writeNode()

This updates documentation of AbstractNormalizedNodeDataOutput
to make it clear that the NormalizedNodeStreamWriter aspect of
it is an implementation detail and that the stream has been
header-initialized by the time any of those methods are called.

The clarification renders calling ensureHeaderWritten() from
writeNode() superfluous, as at that point it is guaranteed to
be a no-op.

Change-Id: I85a5c1304849fe5bc737fa51bd4b40ff2dd8c08c
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>