Remove the custom http plugin part

[soullivaneuh]

This part:

graylog-sdk/src/Graylog.php

Lines 155 to 175 in 51aade2

	/**
	* @param Plugin $plugin
	*/
	private function addPlugin(Plugin $plugin)
	{
	$this->plugins[] = $plugin;
	$this->httpClientModified = true;
	}
	/**
	* @param string $pluginClass
	*/
	private function removePlugin(string $pluginClass)
	{
	foreach ($this->plugins as $p => $plugin) {
	if ($plugin instanceof $pluginClass) {
	unset($this->plugins[$p]);
	$this->httpClientModified = true;
	}
	}
	}

And the custom http plugin client property are completely useless.

The plugin addition must be set by the end user, providing an another PluginClient

Remove this part and implement something like this instead: https://github.com/nexylan/nexycrypt/blob/9df19e89a7d00503f510ded20736287f869b003c/src/NexyCrypt.php#L90-L105

Note: a 0.x release should be made before.