arch: remove stale comments "UAPI Header export list"

These comments are leftovers of commit fcc8487d47 ("uapi: export all
headers under uapi directories").

Prior to that commit, exported headers must be explicitly added to
header-y. Now, all headers under the uapi/ directories are exported.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

arch/ia64/include/uapi/asm/Kbuild

@@ -1,4 +1,3 @@
-# UAPI Header export list
 include include/uapi/asm-generic/Kbuild.asm
 
 generated-y += unistd_64.h