regulator: tps65218: Remove unnecessary regulator_unregister call

Current code uses devm_regulator_register() so the we don't need to explicitly
call regulator_unregister() in .remove.
And then we don't need to save rdev pointer to tps->rdev[id].

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>