kernel: drop patch adding hardcoded kernel compilation flags

1. KCFLAGS should be used for custom flags
2. Optimization flags are arch / SoC specific
3. -fno-reorder-blocks may *worsen* network performace on some SoCs
4. Usage of flags was *reversed* since 5.4 and noone reported that

If we really need custom flags then CONFIG_KERNEL_CFLAGS should get
default value adjusted properly (per target).

Ref: 4e0c54b ("kernel: add support for kernel 5.4")
Link: http://lists.openwrt.org/pipermail/openwrt-devel/2022-June/038853.html
Link: https://patchwork.ozlabs.org/project/openwrt/patch/20190409093046.13401-1-zajec5@gmail.com/
Cc: Felix Fietkau <nbd@nbd.name>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Cc: Rui Salvaterra <rsalvaterra@gmail.com>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>