clear_memory still needed

[timokau]

Most (all?) of our estimators have a clear_memory function that simply resets the estimator while keeping the neural network's weights. The documentation mentions "memory leaks". Are those still an issue? What was the original motivation for adding that function, is there some way to reproduce the issue?

CC @kiudee @prithagupta

[kiudee]

I remember vaguely that this was due to reusing the same session during hyperparameter optimization. The older models would not be removed and accumulate in the memory. @prithagupta do you remember the exact reason?

[timokau]

I propose to remove it in https://github.com/kiudee/cs-ranking/pull/150.