docs(api[toctree]): remove duplicate Testing Utilities entry from api/index.md toctree

why: api/testing/index is now in the root toctree (first-class nav),
so leaving it also in api/index.md's toctree caused Testing Utilities
to appear twice in the /api/ sidebar.

what:
- Remove Testing Utilities <testing/index> from docs/api/index.md toctree
- The Testing grid card section on api/index.md stays (visual only)

Author: tony

docs/api/index.md

@@ -166,5 +166,4 @@ Options <libtmux.options>
 Hooks <libtmux.hooks>
 Constants <libtmux.constants>
 Exceptions <libtmux.exc>
-Testing Utilities <testing/index>
 ```