mfd: qcom_rpm: Remove unused define

This define RPM_SIGNAL probably pertains to the IPC signal which
we ended up fetching from the device tree instead. the define is
unused, remove it.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>