Resolve #463 (Prior Constraints "Simple Example" not working)

This section in the PriorDict sampling seems to be unnecessary since Constraints are checked at many other points in this process. Removing this section fixes the example that was broken by this in #463 (closed)

bilby/core/prior/dict.py

@@ -364,6 +364,12 @@ class PriorDict(dict):
                     return sample
         else:
             needed = np.prod(size)
+            constraint_keys = list()
+            for ii, key in enumerate(keys[-1::-1]):
+                if isinstance(self[key], Constraint):
+                    constraint_keys.append(-ii - 1)
+            for ii in constraint_keys[-1::-1]:
+                del keys[ii]
             all_samples = {key: np.array([]) for key in keys}
             _first_key = list(all_samples.keys())[0]
             while len(all_samples[_first_key]) < needed: