Drop now-superfluous methodNotAllowedHandler

Long ago, the maintainers of gorilla/mux concluded that
it was a "wontfix" to return StatusMethodNotAllowed instead
of StatusNotFound for a method mismatch. Pilosa had a
workaround for this for the most common case (GET requests
to /index/{index}/query), and a TODO to address the other
cases.

While browsing the go-pilosa client code, I noticed that there
is a test for roaring import support which relies on getting
StatusMethodNotAllowed.  But how can this work, if gorilla/mux
doesn't do that?

Answer: They started doing it in mid-2017, apparently:
	https://github.com/gorilla/mux/issues/271

Dropping this code changes the exact text of the message
produced for that one case, but not the status code,
and makes the behavior less confusing.

http/handler.go

@@ -260,20 +260,10 @@ func newRouter(handler *Handler) *mux.Router {
 	router.HandleFunc("/internal/shards/max", handler.handleGetShardsMax).Methods("GET").Name("GetShardsMax") // TODO: deprecate, but it's being used by the client
 	router.HandleFunc("/internal/translate/data", handler.handleGetTranslateData).Methods("GET").Name("GetTranslateData")
 
-	// TODO: Apply MethodNotAllowed statuses to all endpoints.
-	// Ideally this would be automatic, as described in this (wontfix) ticket:
-	// https://github.com/gorilla/mux/issues/6
-	// For now we just do it for the most commonly used handler, /query
-	router.HandleFunc("/index/{index}/query", handler.methodNotAllowedHandler).Methods("GET")
-
 	router.Use(handler.queryArgValidator)
 	return router
 }
 
-func (h *Handler) methodNotAllowedHandler(w http.ResponseWriter, _ *http.Request) {
-	http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
-}
-
 // ServeHTTP handles an HTTP request.
 func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	defer func() {