Use [git.config] for reflog cleaning up (#24958)

Follow
https://github.com/go-gitea/gitea/pull/24860#discussion_r1200589651

Use `[git.config]` for reflog cleaning up, the new options are more
flexible.

*
https://git-scm.com/docs/git-config#Documentation/git-config.txt-corelogAllRefUpdates
*
https://git-scm.com/docs/git-config#Documentation/git-config.txt-gcreflogExpire

## ⚠️ BREAKING

The section `[git.reflog]` is now obsolete and its keys have been moved
to the following replacements:
- `[git.reflog].ENABLED` → `[git.config].core.logAllRefUpdates`
- `[git.reflog].EXPIRATION` → `[git.config].gc.reflogExpire`

modules/git/git.go

@@ -201,23 +201,6 @@ func InitFull(ctx context.Context) (err error) {
 	return syncGitConfig()
 }
 
-func enableReflogs() error {
-	if err := configSet("core.logAllRefUpdates", "true"); err != nil {
-		return err
-	}
-	err := configSet("gc.reflogExpire", fmt.Sprintf("%d", setting.Git.Reflog.Expiration))
-	return err
-}
-
-func disableReflogs() error {
-	if err := configUnsetAll("core.logAllRefUpdates", "true"); err != nil {
-		return err
-	} else if err := configUnsetAll("gc.reflogExpire", ""); err != nil {
-		return err
-	}
-	return nil
-}
-
 // syncGitConfig only modifies gitconfig, won't change global variables (otherwise there will be data-race problem)
 func syncGitConfig() (err error) {
 	if err = os.MkdirAll(HomeDir(), os.ModePerm); err != nil {
@@ -249,16 +232,6 @@ func syncGitConfig() (err error) {
 		return err
 	}
 
-	if setting.Git.Reflog.Enabled {
-		if err := enableReflogs(); err != nil {
-			return err
-		}
-	} else {
-		if err := disableReflogs(); err != nil {
-			return err
-		}
-	}
-
 	if CheckGitVersionAtLeast("2.10") == nil {
 		if err := configSet("receive.advertisePushOptions", "true"); err != nil {
 			return err