[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/git/commit_sha256_test.go

@@ -1,8 +1,6 @@
 // Copyright 2023 The Gitea Authors. All rights reserved.
 // SPDX-License-Identifier: MIT
 
-//go:build !gogit
-
 package git
 
 import (