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.

Change-Id: I76b74c900898670f08abaf1f0e8f4ada2a0962ca
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>
Git-Commit: bd73a328cbb5731000bb3fd876189d9395c0cd61
Git-Repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[bharad@codeaurora.org: resolve trivial merge conflicts]
Signed-off-by: Naitik Bharadiya <bharad@codeaurora.org>