Change signature of StringMethods.rsplit to match pandas

[kroche98]

This removes the nonexistent regex: bool parameter from the StringMethods.rsplit function.

• Closes Incorrect signature for pandas.Series.str.rsplit #1032

I haven't added any tests, as it doesn't seem the parameter types of any string accessor methods are tested.

[Dr-Irv]

I haven't added any tests, as it doesn't seem the parameter types of any string accessor methods are tested.

We do have tests for str.rsplit() in test_series.py. Having said that, adding a test for an incorrect parameter isn't really needed.

[Dr-Irv]

thanks @kroche98