Remove dead visit_exec method (#4943)

* In Python 3 'exec' is just a 'Call' node
diff --git a/pylint/checkers/base.py b/pylint/checkers/base.py
index 12b512f..f37752a 100644
--- a/pylint/checkers/base.py
+++ b/pylint/checkers/base.py
@@ -1386,11 +1386,6 @@
         """
         self._check_unreachable(node)
 
-    @utils.check_messages("exec-used")
-    def visit_exec(self, node):
-        """just print a warning on exec statements"""
-        self.add_message("exec-used", node=node)
-
     def _check_misplaced_format_function(self, call_node):
         if not isinstance(call_node.func, nodes.Attribute):
             return