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/web/webtest/webtest.go

@@ -33,10 +33,8 @@ func NewServer(t testing.TB, routeRegister routing.RouteRegister) *Server {
 	m.Use(func(c *web.Context) {
 		initCtx.Context = c
 		initCtx.Logger = log.New("api-test")
-		c.Map(initCtx)
 
 		c.Req = c.Req.WithContext(ctxkey.Set(c.Req.Context(), initCtx))
-		c.Map(c.Req)
 	})
 
 	m.Use(requestContextMiddleware())
@@ -129,7 +127,9 @@ func requestContextFromRequest(req *http.Request) *models.ReqContext {
 }
 
 func requestContextMiddleware() web.Handler {
-	return func(res http.ResponseWriter, req *http.Request, c *models.ReqContext) {
+	return func(res http.ResponseWriter, req *http.Request) {
+		c := ctxkey.Get(req.Context()).(*models.ReqContext)
+
 		ctx := requestContextFromRequest(req)
 		if ctx == nil {
 			c.Next()
@@ -145,6 +145,5 @@ func requestContextMiddleware() web.Handler {
 		c.RequestNonce = ctx.RequestNonce
 		c.PerfmonTimer = ctx.PerfmonTimer
 		c.LookupTokenErr = ctx.LookupTokenErr
-		c.Map(c)
 	}
 }