mtd: Drop duplicate MTD_PARTITIONS Kconfig option

Commit 9c5b0097 ("Convert CONFIG_MTD_PARTITIONS et al to Kconfig")
introduced a publicly visible Kconfig entry for the
CONFIG_MTD_PARTITIONS option, while the rework on MTD partitioning
was in progress, and we somehow did not notice that the same Kconfig
entry was added by commit 4048a5c5 ("mtd: declare MTD_PARTITIONS
symbol in Kconfig"), but this time as an invisible entry (this can
only be selected by other options).

Keep the non-visible version of this symbol, since MTD_PARTITIONS is
not something the user should be able to enable/disable directly.

Fixes: 4048a5c5 ("mtd: declare MTD_PARTITIONS symbol in Kconfig")
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Jagan Teki <jagan@openedev.com>