kernel: don't force PAHOLE=false

If a specific kernel provider or configuration wants to enable BTF
and pahole analysis, it isn't currently possible due to the explicit
definition to false in the base kernel build arguments.

pahole is now detected by the kernel built itself, so unless
pahole-native is enabled, the result is the same.

If a kernel does require an explicit disable of pahole, it is better
to carry PAHOLE=false in those specific recipes.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>