Makefile.headersinst: cleanup input files

After the last three patches, all exported headers are under uapi/, thus
input-files2 are not needed anymore.
The side effect is that input-files1-name is exactly header-y.

Note also that input-files3-name is genhdr-y.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>