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/gitrepo/config.go

@@ -5,21 +5,11 @@ package gitrepo
 
 import (
 	"context"
-	"strings"
 
 	"code.gitea.io/gitea/modules/git/gitcmd"
 	"code.gitea.io/gitea/modules/globallock"
 )
 
-func GitConfigGet(ctx context.Context, repo Repository, key string) (string, error) {
-	result, _, err := RunCmdString(ctx, repo, gitcmd.NewCommand("config", "--get").
-		AddDynamicArguments(key))
-	if err != nil {
-		return "", err
-	}
-	return strings.TrimSpace(result), nil
-}
-
 func getRepoConfigLockKey(repoStoragePath string) string {
 	return "repo-config:" + repoStoragePath
 }