ARM: dts: Remove led-triggers for RGB LEDs in PMI8998

Currently, RGB LEDs are controlled via led-triggers with fixed events.
Remove these default triggers so that userspace would control these LEDs
for better user experience.

Change-Id: I5f652dd3d0af32cf0f2783b15469d842e228b24a
CRs-Fixed: 2071873
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>