Update plugin.xml

Request remove Plugin.xml: Line[28 ~ 29] since plugin doesn't support iOS which confusing users.

![ios-fileopener](https://f.cloud.github.com/assets/995594/2259326/f8ebe178-9e33-11e3-9d15-c41040342b2e.PNG)

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Discussion: https://getsatisfaction.com/nitobi/topics/_file_opener_plugin_ios_problem