Drop --ignoreskippedcluster option

It's no longer needed.  Clustered VGs are now handled in
the same way as foreign VGs, and as shared VGs that
can't be accessed:

- A command processing all VGs sees a clustered VG,
  prints a message ("Skipping clustered VG foo."),
  skips it, and does not fail.

- A command where the clustered VG is explicitly
  named on the command line, prints a message and fails.
  "Cannot access clustered VG foo, see lvmlockd(8)."

The option is listed in the set of ignored options for
the commands that previously accepted it.  (Removing it
entirely would cause commands/scripts to fail if they
set it.)

lib/commands/toolcontext.h

@@ -144,7 +144,6 @@ struct cmd_context {
 	unsigned report_binary_values_as_numeric:1;
 	unsigned report_mark_hidden_devices:1;
 	unsigned metadata_read_only:1;
-	unsigned ignore_clustered_vgs:1;
 	unsigned threaded:1;			/* set if running within a thread e.g. clvmd */
 	unsigned unknown_system_id:1;
 	unsigned include_historical_lvs:1;	/* also process/report/display historical LVs */