pkg/web: remove dependency injection (#49123)

* pkg/web: store http.Handler internally

* pkg/web: remove injection

Removes any injection code from pkg/web.

It already was no longer functional, as we already only injected into
`http.Handler`, meaning we only inject ctx.Req and ctx.Resp.

Any other types (*Context, *ReqContext) were already accessed using the
http.Request.Context.Value() method.

* *: remove type mappings

Removes any call to the previously removed TypeMapper, as those were
non-functional already.

* pkg/web: remove Context.Invoke

was no longer used outside of pkg/web and also no longer functional

pkg/middleware/request_metrics.go

@@ -56,7 +56,6 @@ func RequestMetrics(features featuremgmt.FeatureToggles) web.Handler {
 		now := time.Now()
 		httpRequestsInFlight.Inc()
 		defer httpRequestsInFlight.Dec()
-		c.Map(c.Req)
 		c.Next()
 
 		handler := "unknown"