Revert "Add np.float16 back"

This reverts commit 1a75bee.

Author: seisman

pygmt/tests/test_clib_to_numpy.py

@@ -92,7 +92,7 @@ def test_to_numpy_python_types(data, expected_dtype):
     pytest.param(np.uint32, np.uint32, id="uint32"),
     pytest.param(np.uint64, np.uint64, id="uint64"),
     # pytest.param(np.ulonglong, np.ulonglong, id="ulonglong"),
-    pytest.param(np.float16, np.float16, id="float16"),
+    # pytest.param(np.float16, np.float16, id="float16"),
     pytest.param(np.float32, np.float32, id="float32"),
     pytest.param(np.float64, np.float64, id="float64"),
     # pytest.param(np.longdouble, np.longdouble, id="longdouble"),