regulator: Remove support for optional supplies in the bulk API

The patch was based on my missinterpretation of the API and only
accidentally worked for me. Let's clean it out to not confuse others.

This reverts commit 3ff3f518.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>