Revert "PERF: perf improvements in drop_duplicates for integer dtyped arrays"

This reverts commit a00c7ea, but leaves new tests and benchmark

Author: evanpw

pandas/core/frame.py

@@ -2994,13 +2994,7 @@ def duplicated(self, subset=None, keep='first'):
         from pandas.hashtable import duplicated_int64, _SIZE_HINT_LIMIT
 
         def f(vals):
-
-            # if we have integers we can directly index with these
-            if com.is_integer_dtype(vals):
-                from pandas.core.nanops import unique1d
-                labels, shape = vals, unique1d(vals)
-            else:
-                labels, shape = factorize(vals, size_hint=min(len(self), _SIZE_HINT_LIMIT))
+            labels, shape = factorize(vals, size_hint=min(len(self), _SIZE_HINT_LIMIT))
             return labels.astype('i8',copy=False), len(shape)
 
         if subset is None: