ARM: OMAP2+: Remove legacy booting support for cm-t35

We've been moving all omap2+ based systems to boot in device tree only
mode for a few years now. Only omap3 has legacy booting support
remaining. Most omap3 boards already have related arch/arm/boot/*.dts*
files for booting with device tree.

This board has support for device tree based booting, and we've been
printing warnings about the legacy booting being deprecated for a
few merge cycles now. Let's attempt to remove the legacy booting
for it.

The reason for removing the legacy booting support now rather than
later is we can simply revert this patch if necessary if we run
into some unexpected issues that are not trivial to fix for the
device tree based booting.

Cc: Dmitry Lifshitz <lifshitz@compulab.co.il>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Tony Lindgren <tony@atomide.com>