mtd: drop of_mtd_parse_partitions()

All users have been converted to call of_mtd_parse_partitions through
parse_mtd_partitions() multiplexer. Drop obsolete API.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Artem Bityutskiy <dedekind1@gmail.com>