TST: remove multiprocess_can_split class variables
these are leftovers from the nose testing and can be removed

```
pandas) bash-3.2$ grep -r multiprocess pandas --include '*.py'
pandas/tests/series/test_apply.py:    _multiprocess_can_split_ = True
pandas/tests/io/parser/multithread.py:from multiprocessing.pool import ThreadPool
pandas/tests/frame/test_apply.py:    _multiprocess_can_split_ = True
pandas/tests/scalar/test_timedelta.py:    _multiprocess_can_split_ = True
pandas/tests/scalar/test_timedelta.py:    _multiprocess_can_split_ = True
pandas/tests/indexes/period/test_period.py:    _multiprocess_can_split_ = True
pandas/tests/indexes/timedeltas/test_timedelta.py:    _multiprocess_can_split_ = True
pandas/tests/indexes/timedeltas/test_timedelta.py:    _multiprocess_can_split_ = True
pandas/tests/indexes/timedeltas/test_indexing.py:    _multiprocess_can_split_ = True
pandas/tests/indexes/timedeltas/test_astype.py:    _multiprocess_can_split_ = True
pandas/tests/indexes/timedeltas/test_setops.py:    _multiprocess_can_split_ = True
pandas/tests/indexes/timedeltas/test_construction.py:    _multiprocess_can_split_ = True
pandas/tests/indexes/timedeltas/test_ops.py:    _multiprocess_can_split_ = True
pandas/tests/indexes/timedeltas/test_timedelta_range.py:    _multiprocess_can_split_ = True
pandas/tests/indexes/timedeltas/test_tools.py:    _multiprocess_can_split_ = True
```