Revert "Added disk-level training"

This reverts commit 7cbcf78a95.

README.md

@@ -160,24 +160,6 @@ Calculate AUC (for one-class MF)
 model.auc(&data, transpose);
 ```
 
-## Disk-Level Training
-
-Train directly from files
-
-```ruby
-model.fit_disk("train.txt");
-model.fit_eval_disk("train.txt", "validate.txt");
-model.cv_disk("train.txt");
-```
-
-Data should be in the format `row_index column_index value`:
-
-```txt
-0 0 5.0
-0 2 3.5
-1 1 4.0
-```
-
 ## Reference
 
 Specify the initial capacity for a matrix