arm: mach-omap2: board-rm680: fix rm680_vemmc regulator constraints

With the commit 75790251 (regulator:
Factor out voltage set operation into a separate function) fixed voltage
regulator setup will fail if there are voltage constraints defined. This
made MMC unusable on this board. Fix by just deleting those redundant
constraints.

Signed-off-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Reviewed-by: Jarkko Nikula <jhnikula@gmail.com>
[tony@atomide.com: updated comments]
Signed-off-by: Tony Lindgren <tony@atomide.com>