Remove unused $persistentDirs

[barryvdh]

I don't think this is used, or am I overlooking something?
It is used here:

magento-composer-installer/src/MagentoHackathon/Composer/Magento/Deploystrategy/Core.php

Lines 130 to 140 in a0765d8

	protected function movePersistentDirectories($sourceDir, $targetDir)
	{
	foreach ($this->persistentDirs as $dir) {
	$source = $sourceDir . self::DS . $dir;
	$target = $targetDir . self::DS . $dir;
	if (file_exists($source)) {
	$this->emptyDir($target);
	$this->fs->copyThenRemove($source, $target);
	}
	}
	}

but that has it's own variable, so this might be confusing.

[Flyingmana]

The whole class is not existent anymore in the 3.x branch, but for the functionality of the core installer they seem to be important, maybe the correct fix for the 2.x versions would be to implement the usage of it then.