Revert "[WFLY-3362] make sure operations get cancelled on channel close"

This reverts commit 89d132d5376f9706aefd34f6eee2bddfb4e83ce6.

Commit introduces a race on the client side during operations like reload/shutdown that result in the channel closing.
Merge pull request #6308 from bstansberry/revert-3362

Revert "[WFLY-3362] make sure operations get cancelled on channel close"