Remove all left over from merge_optimizer

doc/extending/optimization.txt

@@ -212,11 +212,11 @@ optimization you wrote. For example, consider the following:
 Nothing happened here. The reason is: ``add(y, z) != add(y,
 z)``. That is the case for efficiency reasons. To fix this problem we
 first need to merge the parts of the graph that represent the same
-computation, using the ``merge_optimizer`` defined in
+computation, using the ``MergeOptimizer`` defined in
 ``theano.gof.opt``.
 
->>> from theano.gof.opt import merge_optimizer
->>> merge_optimizer.optimize(e)  # doctest: +ELLIPSIS
+>>> from theano.gof.opt import MergeOptimizer
+>>> MergeOptimizer().optimize(e)  # doctest: +ELLIPSIS
 (0, ..., None, None, {}, 1, 0)
 >>> e
 [true_div(mul(*1 -> add(y, z), x), *1)]

theano/sandbox/cuda/opt.py

@@ -104,7 +104,7 @@ optdb.register('gpu_after_fusion',
                'gpu')
 
 # Register merge_optimizer as a global opt
-gpu_optimizer.register('gpu_merge', theano.gof.opt.merge_optimizer,
+gpu_optimizer.register('gpu_merge', theano.gof.opt.MergeOptimizer(),
                        'fast_run', 'fast_compile', final_opt=True)