avocado.plugins.multiplex: Remove format_help

The format_help was used to show "avocado multiplex -h" when
incompatible options were used, but it requires the parser to be stored
and as this is the only plugin doing this, let's just get rid of it
instead fixing the bug.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/plugins/multiplex.py

@@ -90,7 +90,6 @@ class Multiplex(CLICmd):
         elif not args.tree and args.inherit:
             err = "Option --inherit can be only used with --tree"
         if err:
-            view.notify(event="minor", msg=self.parser.format_help())
             view.notify(event="error", msg=err)
             sys.exit(exit_codes.AVOCADO_FAIL)
         try: