ARM: OMAP2+: Remove board file for H4

The old 2420 based H4 is probably not used at all as 2420
was Nokia specific SoC. I have one, but I'm not using it because
of it's large size, and I doubt anybody else is using it either.

We do have minimal omap2420-h4.dts in place, so if anybody wants
more support on H4, patches are welcome.

So let's just remove it as that helps us making mach-omap2 to
be device tree only.

Signed-off-by: Tony Lindgren <tony@atomide.com>