kbuild: drop support of ALTARCH for headers_*

ALTARCH is no longer used by any arch(*) so drop
support for this from Makefile.headerinst

Dropping ALTARCH support simplifies Makefile.headerinst

(*) sparc64 uses it but work is ongoing to drop it
and no furter usage is planned.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: David Miller <davem@davemloft.net>