fix gh-2091

When optimizer=None, we had a crash at Theano import time.

THEANO_FLAGS=optimizer=None,device=gpu python -c 'import theano'
[MAIN] remove class that was never used.
[CRASH] for THEANO_FLAGS=optimizer=merge,device=gpu python -c 'import theano'
Merge pull request #2098 from nouiz/none_opt

[MRG] fix gh-2091