Drop three omap3 legacy board-*.c files

We want to get omap3 moved over to device tree only based booting.

These three board-*.c files seem to be inactive, so let's attempt
to drop them now rather than later. With these dropped, we have
omap3 device tree conversion status in a known state. Then we know
that we have .dts support for the remaining omap3 board-*.c files,
or it's being worked on.

By dropping these files now, we can simply revert them if really
needed for the next few merge cycles if we run into suprises.
Reverting things later on will not be so easily done.
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