kernel: drop unused arch_split_mtd_part()

No single target/arch uses it and most likely there is no need to make
such a potential code target/arch specific.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>

target/linux/generic/pending-4.14/404-mtd-add-more-helper-functions.patch

@@ -11,7 +11,7 @@ Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
 
 --- a/drivers/mtd/mtdpart.c
 +++ b/drivers/mtd/mtdpart.c
-@@ -1232,6 +1232,24 @@ int mtd_is_partition(const struct mtd_in
+@@ -1225,6 +1225,24 @@ int mtd_is_partition(const struct mtd_in
  }
  EXPORT_SYMBOL_GPL(mtd_is_partition);
  
@@ -72,5 +72,5 @@ Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
 +struct mtd_info *mtdpart_get_master(const struct mtd_info *mtd);
 +uint64_t mtdpart_get_offset(const struct mtd_info *mtd);
  uint64_t mtd_get_device_size(const struct mtd_info *mtd);
- extern void __weak arch_split_mtd_part(struct mtd_info *master,
- 				       const char *name, int offset, int size);
+ 
+ #endif