kernel: Remove nvmem hack patch from 5.4

The nvmem framework is now used in net/ethernet/eth.c and the nvmem
sysfs is split into a separate Kconfig option. More work would be needed
to adapt this patch for the broader use. The current patch compiles fine
on ath79, but it breaks the x86 target.

nvmem is also compiled into the kernel for most of our targets for
example ath79 anyway, so patching the kernel to remove it is now harder
and not the case on multiple targets anyway. Instead of making this work
on kernel 5.4 just remove this hack patch.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>