chore: drop the stale tokenizers-cpp submodule entry (#1430)

`.gitmodules` declares three submodules; the tree has two.

| entry | gitlink in tree |
| --- | --- |
| `third-party/tokenizers-cpp` | **absent** |
| `third-party/googletest` | present |
| `packages/react-native-executorch/third-party/common/phonemis` |
present |

The tokenizers migration (#781) removed the gitlink but left the config
entry behind.

Harmless in practice, since `git submodule update` walks the gitlinks in
the index rather than the config, which is why nobody hit it. Still
misleading to anyone reading the file to learn what a checkout needs,
and `create-package.sh` relies on the phonemis entry being accurate.

Verified `git submodule update --init` for both remaining paths still
checks out cleanly after the edit.

Found while auditing root files for staleness ahead of the 0.10 release.
Everything else at the root checked out: `.yarnrc.yml`'s React 19 peer
suppression is still needed (`@signalwire/docusaurus-theme-llms-txt` is
still at `1.0.0-alpha.9` with `react: ^18.0.0` peers), and
`CONTRIBUTING.md`'s paths all still exist.

Author: msluszniak

.gitmodules

@@ -1,6 +1,3 @@
-[submodule "tokenizers-cpp"]
-	path = third-party/tokenizers-cpp
-	url = https://github.com/software-mansion-labs/tokenizers-cpp
 [submodule "third-party/googletest"]
 	path = third-party/googletest
 	url = https://github.com/google/googletest.git