remove flow control mutexes for the sending data

Receiving MAX_{STREAM}_DATA frames and sending data is all done
sequentially, so we don't need a mutex there.

internal/flowcontrol/connection_flow_controller.go

@@ -34,9 +34,6 @@ func NewConnectionFlowController(
 }
 
 func (c *connectionFlowController) SendWindowSize() protocol.ByteCount {
-	c.mutex.RLock()
-	defer c.mutex.RUnlock()
-
 	return c.baseFlowController.sendWindowSize()
 }