examples/plugins/avocado_hello.py: remove leftover "old style" plugin example

This example file should have been remove together with commit
7b969527. Better late than never,
so let's do it now.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

examples/plugins/avocado_hello.py

-from avocado.core.plugins import plugin
-
-
-class HelloWorld(plugin.Plugin):
-
-    """
-    The classical Hello World! plugin example.
-    """
-
-    name = 'hello_world'
-    enabled = True
-
-    def configure(self, parser):
-        """
-        Add the subparser for the 'hello' action.
-        """
-        self.parser = parser.subcommands.add_parser(
-            'hello',
-            help='Hello World! plugin example')
-        super(HelloWorld, self).configure(self.parser)
-
-    def run(self, args):
-        """
-        This method is called whenever we use the command 'hello'.
-        """
-        print(self.__doc__)