OPP: Drop redundant code in _link_required_opps()

Due to that the required-devs for the required OPPs are now always being
assigned, we no longer need the special treatment in _link_required_opps()
for the single PM domain case. Let's therefore drop it.

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20241002122232.194245-8-ulf.hansson@linaro.org