Test case for backend not defined; remove unreachable code

Verify that error is raised in configuration.py if backend is not
defined in config.ini.

Remove unreachable code from instance.py that detects that backend is
not defined.  It hasn't been reachable since version 1.6.1 because the
configuration.py module made the backend option mandatory. Hence
configuration.py raises the error and never passes control back to the
unreachable code.

Author: rouilj

roundup/instance.py

@@ -80,17 +80,6 @@ def __init__(self, tracker_home, optimize=0):
 
         rdbms_backend = self.config.RDBMS_BACKEND
 
-        # TODO: Remove in v1.7
-        # Provide some backwards compatability for existing Roundup instances
-        # that still define the backend type in 'db/backend_name' and warn the
-        # users they need to update their config.ini. Note that a missing
-        # rdbms backend causes the config to throw an error, so this may
-        # not be possible.
-        if rdbms_backend == '':
-            raise TrackerError("database backend not found in config.ini.\n"
-                   "Old style `backend_name` in db directory not supported\n"
-                   "See doc/upgrading.txt for required steps.")
-
         self.backend = backends.get_backend(rdbms_backend)
 
         if self.optimize: