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.

Sources/NIO/ChannelPipeline.swift

@@ -285,13 +285,8 @@ public final class ChannelPipeline: ChannelInvoker {
         let context = ChannelHandlerContext(name: name ?? nextName(), handler: handler, pipeline: self)
         operation(context, relativeContext)
 
-        do {
-            try context.invokeHandlerAdded()
-            return .success(())
-        } catch let err {
-            removeHandlerFromPipeline(context: context, promise: nil)
-            return .failure(err)
-        }
+        context.invokeHandlerAdded()
+        return .success(())
     }
 
     /// Synchronously add a single new `ChannelHandlerContext` after one that currently exists in the
@@ -505,12 +500,8 @@ public final class ChannelPipeline: ChannelInvoker {
             nextCtx.prev = prevCtx
         }
 
-        do {
-            try context.invokeHandlerRemoved()
-            promise?.succeed(())
-        } catch let err {
-            promise?.fail(err)
-        }
+        context.invokeHandlerRemoved()
+        promise?.succeed(())
 
         // We need to keep the current node alive until after the callout in case the user uses the context.
         context.next = nil
@@ -1509,13 +1500,13 @@ public final class ChannelHandlerContext: ChannelInvoker {
         }
     }
 
-    fileprivate func invokeHandlerAdded() throws {
+    fileprivate func invokeHandlerAdded() {
         self.eventLoop.assertInEventLoop()
 
         handler.handlerAdded(context: self)
     }
 
-    fileprivate func invokeHandlerRemoved() throws {
+    fileprivate func invokeHandlerRemoved() {
         self.eventLoop.assertInEventLoop()
         guard !self.removeHandlerInvoked else {
             return