package/mtd: make ubihealthd independent of ubifs

Since version 2.1.3 ubihealthd can be enabled without of ubifs-utils.

This also fixes usability of enabling BR2_PACKAGE_MTD_UBIHEALTHD.
BR2_PACKAGE_MTD_UBIFS_UTILS is a blind option. The only way to enable it
is to enable BR2_PACKAGE_MTD_MKFSUBIFS that selects it. ubihealthd
dependency on BR2_PACKAGE_MTD_UBIFS_UTILS makes enabling it unintuitive.

Cc: Markus Mayer <mmayer@broadcom.com>
Cc: Matt Weber <matthew.weber@collins.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>