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/services/serviceaccounts/api/api_test.go

@@ -238,10 +238,7 @@ func setupTestServer(t *testing.T, svc *tests.ServiceAccountMock,
 			SignedInUser: signedUser,
 			Logger:       log.New("serviceaccounts-test"),
 		}
-		c.Map(ctx)
-
 		c.Req = c.Req.WithContext(ctxkey.Set(c.Req.Context(), ctx))
-		c.Map(c.Req)
 	})
 	a.RouterRegister.Register(m.Router)
 	return m, a