patches/openwrt: drop patch to fix kernel not locate root FS on microSD

This patch was necessary only on OpenWrt 19.07, because the kernel
config CONFIG_OF_DMA_DEFAULT_COHERENT was disabled, but it should be
enabled by default [1] [2].

I can see that in OpenWrt 21.02 and newer versions the config is already
enabled, thus patch is no longer needed:

root@turris:/# zcat /proc/config.gz | grep CONFIG_OF_DMA_DEFAULT_COHERENT
CONFIG_OF_DMA_DEFAULT_COHERENT=y

[1] https://elixir.bootlin.com/linux/v5.18.2/source/drivers/of/address.c#L1055
[2] https://elixir.bootlin.com/linux/v5.18.2/source/arch/powerpc/Kconfig#L261