qemu: remove CONFIG_JFFS2_FS=n from kernel configuration

Commit f1ca9916 ("qemu: disable kernel CONFIG_JFFS2_FS") was a
workaround to an issue that does not seem to be reproducible anymore.
That was more than five years ago, and the root cause was never clearly
identified. Many things have changed since.
I did not meet any problem running "make check" with CONFIG_JFFS2_FS=y
on my local development machine (it is enabled by default in the Linux
kernel's arch/arm/configs/qemu_defconfig). Therefore it seems safe to
remove this CONFIG_JFFS2_FS=n.

Signed-off-by: Jerome Forissier <jerome@forissier.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Etienne Carriere <etienne.carriere@linaro.org>