ARM: ux500: regulators: Remove dead code for SD-card regulator

The signal-voltage regulator is handled through a gpio regulator
configured in DT. Remove the old dead code.

Cc: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>