Drop 'not in' and 'is not' tests from HACKING.rst

These rules were removed from hacking in Ic01e55781d36328a03692418537b642f1f719d1b
Since they are now covered directly in pep8. HACKING.rst is a list of
rules that augment pep8, and should not restate any pep8 rules.

Change-Id: Idb8298f7ff8f09693e98f4ee8b5a25b0b13a5951
Merge "Drop 'not in' and 'is not' tests from HACKING.rst"