Enable partial clone by default (#18195)

- Enable partial clones(which are by default disabled from git) by
default, unless configured otherwise.
- Resolves #18190

doc/advanced/clone-filter.en-us.md

@@ -27,36 +27,9 @@ on the client is at least the same as on the server (or later). Login to
 Gitea server as admin and head to Site Administration -> Configuration to
 see Git version of the server.
 
-By default, clone filters are disabled, which cause the server to ignore
-`--filter` option.
+By default, clone filters are enabled, unless `DISABLE_PARTIAL_CLONE` under
+`[git]` is set to `true`.
 
-To enable clone filters on per-repo basis, edit the repo's `config` on
-repository location. Consult `ROOT` option on `repository` section of
-Gitea configuration (`app.ini`) for the exact location. For example, to
-enable clone filters for `some-repo`, edit
-`/var/gitea/data/gitea-repositories/some-user/some-repo.git/config` and add:
-
-```ini
-[uploadpack]
-	allowfilter = true
-```
-
-To enable clone filters globally, add that config above to `~/.gitconfig`
-of user that run Gitea (for example `git`).
-
-Alternatively, you can use `git config` to set the option.
-
-To enable for a specific repo:
-
-```bash
-cd /var/gitea/data/gitea-repositories/some-user/some-repo.git
-git config --local uploadpack.allowfilter true
-```
-To enable globally, login as user that run Gitea and:
-
-```bash
-git config --global uploadpack.allowfilter true
-```
 
 See [GitHub blog post: Get up to speed with partial clone](https://github.blog/2020-12-21-get-up-to-speed-with-partial-clone-and-shallow-clone/)
 for common use cases of clone filters (blobless and treeless clones), and