Remove singleton pattern in base_generator

A generator doesn't need to be a singleton and this causes problems
when executing unit tests. This code was simply a left-over.

Change-Id: Ieaf71bf824a630c75c95b190f34cc55a812d7fc0
Closes-Bug: #1300675
diff --git a/tempest/common/generator/base_generator.py b/tempest/common/generator/base_generator.py
index 95d50e2..87d65d0 100644
--- a/tempest/common/generator/base_generator.py
+++ b/tempest/common/generator/base_generator.py
@@ -87,12 +87,6 @@
         "additionalProperties": False,
     }
 
-    def __new__(cls, *args, **kwargs):
-        if not cls._instance:
-            cls._instance = super(BasicGeneratorSet, cls).__new__(cls, *args,
-                                                                  **kwargs)
-        return cls._instance
-
     def __init__(self):
         self.types_dict = {}
         for m in dir(self):