Remove descriptions of nonexistent hacking rules

N321, N328, N329, N330 hacking rules have been removed
since I9c334162fe1799e7b24563fdc11256b91bbafc9f.
However the descriptions are still in HACKING.rst.
So remove them.
The rule number N307 is missing in HACKING.rst.
So add it.

Change-Id: I868c421a0f5a3329ab36f786f8519accae623f1a
Closes-Bug: #1841400

HACKING.rst

@@ -8,7 +8,7 @@ Nova Style Commandments
 Nova Specific Commandments
 ---------------------------
 
-- ``nova.db`` imports are not allowed in ``nova/virt/*``
+- [N307] ``nova.db`` imports are not allowed in ``nova/virt/*``
 - [N309] no db session in public API methods (disabled)
   This enforces a guideline defined in ``oslo.db.sqlalchemy.session``
 - [N310] timeutils.utcnow() wrapper must be used instead of direct calls to
@@ -28,16 +28,12 @@ Nova Specific Commandments
 - [N319] Validate that debug level logs are not translated.
 - [N320] Setting CONF.* attributes directly in tests is forbidden. Use
   self.flags(option=value) instead.
-- [N321] Validate that LOG messages, except debug ones, have translations
 - [N322] Method's default argument shouldn't be mutable
 - [N323] Ensure that the _() function is explicitly imported to ensure proper translations.
 - [N324] Ensure that jsonutils.%(fun)s must be used instead of json.%(fun)s
 - [N325] str() and unicode() cannot be used on an exception.  Remove use or use six.text_type()
 - [N326] Translated messages cannot be concatenated.  String should be included in translated message.
 - [N327] Do not use xrange(). xrange() is not compatible with Python 3. Use range() or six.moves.range() instead.
-- [N328] Validate that LOG.info messages use _LI.
-- [N329] Validate that LOG.exception messages use _LE.
-- [N330] Validate that LOG.warning and LOG.warn messages use _LW.
 - [N332] Check that the api_version decorator is the first decorator on a method
 - [N334] Change assertTrue/False(A in/not in B, message) to the more specific
   assertIn/NotIn(A, B, message)