Removing unused Http2PrefaceHandler.

Motivation:

All of the functionality has been merged into
AbstractHttp2ConnectionHandler.

Modifications:

Removed the Http2PrefaceHandler/Test and all uses.

Result:

Code cleaned up a bit.
Changed Http2ClientConnectionHandler to not update reader index.

Motivation:
When running the Http2Client the data returned from the server, the
"Hello World" string, is supposed to be printed but instead the following
is displayed:
Received message:

The reason is that the following line increases the ByteBuf's readerIndex:
collectedData.writeBytes(data);

Modifications:
Changing the above mentioned method to use a writeBytes call that does
not increase the readerIndex:
collectedData.writeBytes(data, data.readerIndex(), data.readableBytes());

Result:
The example now logs the correct data sent from the server:
Received message: Hello World