PERF: remove useless overrides (#21523)

closes #21522
(cherry picked from commit ea54d39)

Author: toobaz

pandas/core/indexes/multi.py

@@ -852,14 +852,6 @@ def _has_complex_internals(self):
         # to disable groupby tricks
         return True
 
-    @cache_readonly
-    def is_monotonic(self):
-        """
-        return if the index is monotonic increasing (only equal or
-        increasing) values.
-        """
-        return self.is_monotonic_increasing
-
     @cache_readonly
     def is_monotonic_increasing(self):
         """
@@ -887,10 +879,6 @@ def is_monotonic_decreasing(self):
         # monotonic decreasing if and only if reverse is monotonic increasing
         return self[::-1].is_monotonic_increasing
 
-    @cache_readonly
-    def is_unique(self):
-        return not self.duplicated().any()
-
     @cache_readonly
     def _have_mixed_levels(self):
         """ return a boolean list indicated if we have mixed levels """