OPP: Drop redundant *_opp_attach|detach_genpd()

All users of *_opp_attach|detach_genpd(), have been converted to use
dev|devm_pm_domain_attach|detach_list(), hence let's drop it along with its
corresponding exported functions.

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20241002122232.194245-12-ulf.hansson@linaro.org