Deprecate reg:linear' in favor of reg:squarederror'. (#4267)

* Deprecate `reg:linear' in favor of `reg:squarederror'.
* Replace the use of `reg:linear'.
* Replace the use of `silent`.

tests/python/test_updaters.py

@@ -33,7 +33,7 @@ class TestUpdaters(unittest.TestCase):
                           'max_bin': [2, 256],
                           'grow_policy': ['depthwise', 'lossguide'],
                           'max_leaves': [64, 0],
-                          'silent': [1]}
+                          'verbosity': [0]}
         for param in parameter_combinations(variable_param):
             result = run_suite(param)
             assert_results_non_increasing(result, 1e-2)
@@ -45,7 +45,7 @@ class TestUpdaters(unittest.TestCase):
         ag_param = {'max_depth': 2,
                     'tree_method': 'hist',
                     'eta': 1,
-                    'silent': 1,
+                    'verbosity': 0,
                     'objective': 'binary:logistic',
                     'eval_metric': 'auc'}
         hist_res = {}