[CHORE] Drop go-git support

See https://codeberg.org/forgejo/discussions/issues/164 for the
rationale and discussion of this change.

Everything related to the `go-git` dependency is dropped (Only a single
instance is left in a test file to test for an XSS, it requires crafting
an commit that Git itself refuses to craft). `_gogit` files have
been removed entirely, `go:build: !gogit` is removed, `XXX_nogogit.go` files
either have been renamed or had their code being merged into the
`XXX.go` file.

modules/gitrepo/walk.go

@@ -1,8 +1,6 @@
 // Copyright 2024 The Gitea Authors. All rights reserved.
 // SPDX-License-Identifier: MIT
 
-//go:build !gogit
-
 package gitrepo
 
 import (

modules/gitrepo/walk_gogit.go

@@ -1,40 +0,0 @@
-// Copyright 2024 The Gitea Authors. All rights reserved.
-// SPDX-License-Identifier: MIT
-
-//go:build gogit
-
-package gitrepo
-
-import (
-	"context"
-
-	"github.com/go-git/go-git/v5/plumbing"
-)
-
-// WalkReferences walks all the references from the repository
-// refname is empty, ObjectTag or ObjectBranch. All other values should be treated as equivalent to empty.
-func WalkReferences(ctx context.Context, repo Repository, walkfn func(sha1, refname string) error) (int, error) {
-	gitRepo := repositoryFromContext(ctx, repo)
-	if gitRepo == nil {
-		var err error
-		gitRepo, err = OpenRepository(ctx, repo)
-		if err != nil {
-			return 0, err
-		}
-		defer gitRepo.Close()
-	}
-
-	i := 0
-	iter, err := gitRepo.GoGitRepo().References()
-	if err != nil {
-		return i, err
-	}
-	defer iter.Close()
-
-	err = iter.ForEach(func(ref *plumbing.Reference) error {
-		err := walkfn(ref.Hash().String(), string(ref.Name()))
-		i++
-		return err
-	})
-	return i, err
-}