types: remove unused git_merge_result `git_merge_result` is currently unused in the codebase and generates a blank page in the [documentation](https://libgit2.github.com/libgit2/#HEAD/type/git_merge_result).
diff --git a/include/git2/types.h b/include/git2/types.h
index 8d9a947..ad34e01 100644
--- a/include/git2/types.h
+++ b/include/git2/types.h
@@ -181,9 +181,6 @@ typedef struct git_transaction git_transaction;
/** Annotated commits, the input to merge and rebase. */
typedef struct git_annotated_commit git_annotated_commit;
-/** Merge result */
-typedef struct git_merge_result git_merge_result;
-
/** Representation of a status collection */
typedef struct git_status_list git_status_list;