mtd: diskonchip: remove two-phase partitioning / registration

It is a Bad Idea (TM) to call mtd_device_register() or
mtd_device_parse_register() twice on the same master MTD. Among other
things, it makes partition overrides (e.g., cmdlinepart) much more
difficult.

Since commit 727dc612 ("mtd: part: Create the master device node
when partitioned"), we now have a config option that accomplishes the
same purpose as the double-registration done in diskonchip.c -- it
forces the master MTD to *always* be registered, while partitions may
optionally show up in addition. Eventually, we might like to make
CONFIG_MTD_PARTITIONED_MASTER into the default, but this could be
disruptive to user-space expectations of MTD numbering, so we'll take
that slowly.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Alexander Shiyan <shc_work@mail.ru>