Remove the onrequest hook

It was redundant with middlewares. Allows us to save a few operations
for every incoming requests.

ROADMAP.md

@@ -45,9 +45,6 @@ the halt tuple. The error tuple will therefore be removed.
 
 ### Hooks
 
-The `onrequest` hook will be removed. It can easily be replaced
-by a middleware.
-
 The interface of the `onresponse` hook will change. There has
 been a number of issues and added complexity with the current
 interface that warrant fixing. The main problem is that the

doc/src/guide/hooks.ezdoc

@@ -1,45 +1,7 @@
 ::: Hooks
 
-Cowboy provides two hooks. `onrequest` is called once the request
-line and headers have been received. `onresponse` is called just
-before sending the response.
-
-:: Onrequest
-
-The `onrequest` hook is called as soon as Cowboy finishes fetching
-the request headers. It occurs before any other processing, including
-routing. It can be used to perform any modification needed on the
-request object before continuing with the processing. If a reply is
-sent inside this hook, then Cowboy will move on to the next request,
-skipping any subsequent handling.
-
-This hook is a function that takes a request object as argument,
-and returns a request object. This function MUST NOT crash. Cowboy
-will not send any reply if a crash occurs in this function.
-
-You can specify the `onrequest` hook when creating the listener,
-inside the request options.
-
-``` erlang
-cowboy:start_http(my_http_listener, 100,
-    [{port, 8080}],
-    [
-        {env, [{dispatch, Dispatch}]},
-        {onrequest, fun ?MODULE:debug_hook/1}
-    ]
-).
-```
-
-The following hook function prints the request object everytime a
-request is received. This can be useful for debugging, for example.
-
-``` erlang
-debug_hook(Req) ->
-    erlang:display(Req),
-    Req.
-```
-
-Make sure to always return the last request object obtained.
+Hooks allow the user to customize Cowboy's behavior during specific
+operations.
 
 :: Onresponse
 
@@ -48,9 +10,9 @@ to the socket. It can be used for the purposes of logging responses,
 or for modifying the response headers or body. The best example is
 providing custom error pages.
 
-Note that like the `onrequest` hook, this function MUST NOT crash.
-Cowboy may or may not send a reply if this function crashes. If a reply
-is sent, the hook MUST explicitly provide all headers that are needed.
+Note that this function MUST NOT crash. Cowboy may or may not send a
+reply if this function crashes. If a reply is sent, the hook MUST
+explicitly provide all headers that are needed.
 
 You can specify the `onresponse` hook when creating the listener.