Remove oniguruma lines from CONTRIBUTING

Author: nikic

CONTRIBUTING.md

@@ -126,8 +126,6 @@ locations.
        └─ ...
     └─ mbstring/
        ├─ libmbfl/                  # Forked and maintained in php-src
-       ├─ oniguruma/                # Bundled https://github.com/kkos/oniguruma
-       ├─ oniguruma.patch           # Modifications patch from upstream oniguruma
        ├─ unicode_data.h            # Generated by `ext/mbstring/ucgendat/ucgendat.php`
        └─ ...
     └─ pcre/