github: remove misleading feature template

The QGC Project doesn't take feature request trough Github, instead we
use the official Forums http://discuss.px4.io/c/qgroundcontrol

.github/ISSUE_TEMPLATE/feature_request.md

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-name: Feature request
-about: Suggest an idea for this project
-labels: 
-
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-
-Please use [the forums](http://discuss.px4.io/c/qgroundcontrol) for new feature requests. You can cross-link the thread in a new issue. However, due to the high amount of user feedback expect the QGC maintainer team to close feature requests that can't be implemented relatively quickly.