Remove a redundant SimpleFuture

There was 2 SimpleFuture classes, one in com.flowpowered.commons and one
in com.flowpowered.commons.future . The latter one was the only class in
it's package, and had tabs instead of spaces. Other than that they were 
the same.

There's no need to have them both. Remove the one from
com.flowpowered.commons.future .