refactor!: remove go-git backend (#39487)

Removes the go-git backend so every build uses the git CLI backend. Its
Windows performance advantage is gone, it lacks SHA-256 support, and it
breaks repositories on Windows.

The performance changes moved to
https://github.com/go-gitea/gitea/pull/39526.

Fixes https://github.com/go-gitea/gitea/issues/38359
Fixes https://github.com/go-gitea/gitea/issues/34694

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

docs/build-source.md

@@ -25,7 +25,6 @@ Depending on requirements, the following build tags can be included.
 - `bindata`: Build a single monolithic binary, with all assets included. Required for distribution and production build.
 - `pam`: Enable support for PAM (Linux Pluggable Authentication Modules).
   Can be used to authenticate local users or extend authentication to methods available to PAM.
-- `gogit`: (EXPERIMENTAL) Use go-git variants of Git commands.
 
 To include all assets, use the `bindata` tag:
 
@@ -33,13 +32,6 @@ To include all assets, use the `bindata` tag:
 TAGS="bindata" make build
 ```
 
-Tag `gogit` is used to try to resolve some Windows-specific performance problems, POSIX systems don't need it.
-You can build a Windows binary by:
-
-```bash
-GOOS=windows TAGS="bindata gogit" make build
-```
-
 ## Changing default paths
 
 Gitea will search for a number of things from the _`CustomPath`_.