Remove untranslatable on_date key (#24106)

- Follows #23988 
- Fixes: #24074 by removing this key

GitHub's `relative-time` elements allow us to force their rendering to
`auto`, `past`, or `future` tense. We will never show an absolute date
`on ...` in `TimeSince`

## Before

![image](https://user-images.githubusercontent.com/20454870/231735872-048c7bf3-6aa1-4113-929d-75a985c9922c.png)

## After

![image](https://user-images.githubusercontent.com/20454870/231736116-6ad47b63-77f4-4d3f-82a2-ee9a46ba2bd1.png)

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/web/devtest/devtest.go

@@ -7,6 +7,7 @@ import (
 	"net/http"
 	"path"
 	"strings"
+	"time"
 
 	"code.gitea.io/gitea/modules/base"
 	"code.gitea.io/gitea/modules/context"
@@ -32,5 +33,14 @@ func List(ctx *context.Context) {
 }
 
 func Tmpl(ctx *context.Context) {
+	now := time.Now()
+	ctx.Data["TimeNow"] = now
+	ctx.Data["TimePast5s"] = now.Add(-5 * time.Second)
+	ctx.Data["TimeFuture5s"] = now.Add(5 * time.Second)
+	ctx.Data["TimePast2m"] = now.Add(-2 * time.Minute)
+	ctx.Data["TimeFuture2m"] = now.Add(2 * time.Minute)
+	ctx.Data["TimePast1y"] = now.Add(-1 * 366 * 86400 * time.Second)
+	ctx.Data["TimeFuture1y"] = now.Add(1 * 366 * 86400 * time.Second)
+
 	ctx.HTML(http.StatusOK, base.TplName("devtest"+path.Clean("/"+ctx.Params("sub"))))
 }