mtd: partitions: remove sysfs files when deleting all master's partitions

When support for sysfs "offset" file was added it missed to update the
del_mtd_partitions function. It deletes partitions just like
mtd_del_partition does so both should also take care of removing sysfs
files.

This change moves sysfs_remove_files call to the shared function to fix
this issue.

Fixes: a62c24d7 ("mtd: part: Add sysfs variable for offset of partition")
Cc: Dan Ehrenberg <dehrenberg@chromium.org>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>