Revert "kernel: rewrite run_parsers_by_type() to use add_mtd_partitions()"

Revert "kernel: rewrite run_parsers_by_type() to use add_mtd_partitions()"

This reverts commit 15a0701cdde8eeae2a54880b813cdb8cdc09a384.

It was reported that this patch breaks on some cases the JFFS2 overlay
filesystem on targets still using kernel 4.14. This includes ar71xx,
where spurious erase of the ART was reported.

Revert this commit for now. Re-adding should probaby also be done for
every currently supported kernel version.

Ref: FS#2837 FS#2862 FS#2864
Signed-off-by: David Bauer <mail@david-bauer.net>