soc/tegra: pmc: Drop the ->opp_to_performance_state() callback

Since commit 7c41cdcd

 ("OPP: Simplify the over-designed pstate <->
level dance"), there is no longer any need for genpd providers to assign
the ->opp_to_performance_state(), hence let's drop it.

Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Jonathan Hunter <jonathanh@nvidia.com>
Cc: linux-tegra@vger.kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>