Remove dead code identified by deadcode tool (#37271)

Ran [`deadcode`](https://pkg.go.dev/golang.org/x/tools/cmd/deadcode)
(`-test ./...`) to find functions, methods and error types unreachable
from any call path (including tests), and removed the truly-dead ones.

Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>

modules/eventsource/event.go

@@ -7,7 +7,6 @@ import (
 	"bytes"
 	"fmt"
 	"io"
-	"strings"
 	"time"
 
 	"code.gitea.io/gitea/modules/json"
@@ -110,9 +109,3 @@ func (e *Event) WriteTo(w io.Writer) (int64, error) {
 
 	return sum, err
 }
-
-func (e *Event) String() string {
-	buf := new(strings.Builder)
-	_, _ = e.WriteTo(buf)
-	return buf.String()
-}