DISK: Do not allow to emit any "footers"

This is the initial iteration of cleaning up simh of appending the
"footers" to disk images:  it does not allow to emit any new "footers",
yet it does not change the behavior as to how any existing "footers"
are treated internally.  Also, even though sim_disk.c currently builds
the "footer" for any attached disk container, with this patch, it is
only kept in-memory, and never gets written to (appended / updated)
the actual container.

sim_disk.c

@@ -2439,6 +2439,8 @@ time_t now = time (NULL);
 t_offset total_sectors;
 t_offset highwater;
 
+return SCPE_OK;
+
 if ((dptr = find_dev_from_unit (uptr)) == NULL)
     return SCPE_NOATT;
 if (uptr->flags & UNIT_RO)
@@ -2502,6 +2504,8 @@ t_offset total_sectors;
 t_offset highwater;
 t_offset footer_highwater;
 
+return SCPE_OK;
+
 if ((dptr = find_dev_from_unit (uptr)) == NULL)
     return SCPE_NOATT;
 if (uptr->flags & UNIT_RO)