Remove duplicated thread-pool API (#308)

Removing an old and broken thread-pool module: 
```python
kvikio.thread_pool.num_threads_reset()
kvikio.thread_pool.get_num_threads()
```

Use the default module instead:
```python
kvikio.defaults.num_threads_reset()
kvikio.defaults.get_num_threads()
```

Authors:
  - Mads R. B. Kristensen (https://github.com/madsbk)

Approvers:
  - Lawrence Mitchell (https://github.com/wence-)

URL: #308

python/kvikio/defaults.py

@@ -88,7 +88,9 @@ def num_threads_reset(nthreads: int) -> None:
     Parameters
     ----------
     nthreads : int
-        The number of threads to use.
+        The number of threads to use. The default value can be specified by setting
+        the `KVIKIO_NTHREADS` environment variable. If not set, the default value
+        is 1.
     """
     libkvikio.thread_pool_nthreads_reset(nthreads)