Undo 24baa9f, allow to skip Theano legacy benchmark with environment variable

24baa9f breaks the FFT benchmark and effectively does the GpuCorrMM benchmark three times in a row. This PR undoes 24baa9f and instead allows to skip the Theano legacy benchmark with:
```
SKIP_LEGACY=1 python pylearn2_benchmark.py
```

/Edit: It also removes the `pycuda.driver.Context.pop()` call because in the meantime this has been added to Theano -- now `pycuda.driver.Context.pop()` throws an error message instead of suppressing one :)