kernel: remove LINUX_4_9 dependency of kmod-dax

kernel: remove LINUX_4_9 dependency of kmod-dax

This patch resolves recursive dependency warning on a feed package:

$ make defconfig
Collecting package info: done
tmp/.config-package.in:104721:error: recursive dependency detected!
For a resolution refer to Documentation/kbuild/kconfig-language.txt
subsection "Kconfig recursive dependency limitations"
tmp/.config-package.in:104721:symbol PACKAGE_nfs-kernel-server depends on NFS_KERNEL_SERVER_V4
For a resolution refer to Documentation/kbuild/kconfig-language.txt
subsection "Kconfig recursive dependency limitations"
feeds/packages/net/nfs-kernel-server/Config.in:4:symbol NFS_KERNEL_SERVER_V4 depends on PACKAGE_nfs-kernel-server
#
# configuration written to .config
#

19.07 branch uses kernel 4.14 only, so CONFIG_LINUX_4_9 symbol is not
needed anyway.

Ref: https://github.com/openwrt/packages/issues/10490

Signed-off-by: Sungbo Eo <mans0n@gorani.run>