Makefile.headersinst: remove destination-y option

This option was added in commit c7bb349e ("kbuild: introduce destination-y
for exported headers") but never used in-tree.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>