Remove processing of now-nonexistent flush promises. (#6)
Remove processing of now-nonexistent flush promises. (#6)

Motivation:

There are no more flush promises in NIO, so we don't need to bend
over backwards to track them appropriately.

Modifications:

Remove the code that handles flush promises. Simplify.

Result:

Simpler, easier to follow code for buffered writes.
apple/swift-nio-ssl,apple/swift-nio-ssl,apple/swift-nio-ssl,apple/swift-nio-ssl,apple/swift-nio-ssl,apple/swift-nio-ssl