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/infra/usagestats/service/api_test.go

@@ -103,9 +103,6 @@ func contextProvider(tc *testContext) web.Handler {
 			SkipCache:    true,
 			Logger:       log.New("test"),
 		}
-		c.Map(reqCtx)
-
 		c.Req = c.Req.WithContext(ctxkey.Set(c.Req.Context(), reqCtx))
-		c.Map(c.Req)
 	}
 }