package/linux-headers: really drop the 4.17 choice

package/linux-headers: really drop the 4.17 choice

Currently, the build fails when the kernel headers 4.17 are selected:

    package/linux-headers/linux-headers.mk:149: *** LINUX_HEADERS_SITE
    cannot be empty when LINUX_HEADERS_SOURCE is not.  Stop.

This is caused by an hickup during the last merge of the next branch,
where the value for 4.17 got dropped from BR2_DEFAULT_KERNEL_HEADERS,
but the choice entry was left in.

The correct fix against that hickup would be to restore the 4.17 value.
But since 4.17 is no longer maintained, the proper fix is to really drop
4.17 altogether.

For good this time! ;-)

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>