regulator: max8997: Remove empty set_suspend_enable callback implementation

Since commit 8ac0e95d "regulator: core: Support setting suspend_[mode|voltage]
if set_suspend_[en|dis]able is NULL", now the regulator core can properly
handle the case set_suspend_enable callback is NULL.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>