omap: rx51_defconfig: Remove duplicate phonet

Phonet is enabled by the commit bce54fed
and this duplicate gives a warning when doing make rx51_defconfig.

Signed-off-by: Jarkko Nikula <jhnikula@gmail.com>
Acked-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>