opened hook was never used, let's remove it.

lib/protocol/http2/stream.rb

@@ -221,19 +221,13 @@ def send_data(*arguments, **options)
 				end
 			end
 
-			# The stream has been opened.
-			def opened(error = nil)
-			end
-
 			def open!
 				if @state == :idle
 					@state = :open
 				else
 					raise ProtocolError, "Cannot open stream in state: #{@state}"
 				end
 
-				self.opened
-
 				return self
 			end