feat: remove the legacy TEST_CONFLICTING_PATCHES_WITH_GIT_APPLY setting (#7745)

- The way of doing conflict testing via `git apply` stems from Gogs, it was replaced in Gitea 1.18 by `git read-tree -m` which uses 3-way merge [^0]. The option to disable the fallback `git apply` was introduced in Gitea 1.19 and enabled by default [^1].
- Given it was mostly kept just in case `git read-tree -m` was shown to be unreliable and it has been sufficiently battle tested with no known issues (in Forgejo), it's time to remove this way of conflict testing. I am not aware of anyone using this option or having any benefits over a 3-way merge via `git read-tree -m`.

[^0]: https://github.com/go-gitea/gitea/pull/18004
[^1]: https://github.com/go-gitea/gitea/pull/22130

<!--start release-notes-assistant-->

## Release notes
<!--URL:https://codeberg.org/forgejo/forgejo-->
- Breaking features
  - [PR](https://codeberg.org/forgejo/forgejo/pulls/7745): <!--number 7745 --><!--line 0 --><!--description cmVtb3ZlIHRoZSBsZWdhY3kgYFRFU1RfQ09ORkxJQ1RJTkdfUEFUQ0hFU19XSVRIX0dJVF9BUFBMWWAgc2V0dGluZw==-->remove the legacy `TEST_CONFLICTING_PATCHES_WITH_GIT_APPLY` setting<!--description-->
<!--end release-notes-assistant-->

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7745
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

modules/setting/repository.go

@@ -95,7 +95,6 @@ var (
 			DefaultUpdateStyle                       string
 			PopulateSquashCommentWithCommitMessages  bool
 			AddCoCommitterTrailers                   bool
-			TestConflictingPatchesWithGitApply       bool
 			RetargetChildrenOnMerge                  bool
 		} `ini:"repository.pull-request"`
 
@@ -226,7 +225,6 @@ var (
 			DefaultUpdateStyle                       string
 			PopulateSquashCommentWithCommitMessages  bool
 			AddCoCommitterTrailers                   bool
-			TestConflictingPatchesWithGitApply       bool
 			RetargetChildrenOnMerge                  bool
 		}{
 			WorkInProgressPrefixes: []string{"WIP:", "[WIP]"},