Remove error handling for adding/removing handlers functions (#1712)
Remove error handling for adding/removing handlers functions (#1712)

Motivation:

The handlerAdded and handlerRemoved functions are not throwing.
I can't spot anyway way and exception could actually happen
so no point in catering for the case when one does.

Modifications:

Remove possibility that an exception was thrown when adding or
removing handlers as none of the called functions are throwing.

Result:

Fewer lines of code, less complexity.
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio