regulator: devres: remove devm_regulator_unregister() function

regulator: devres: remove devm_regulator_unregister() function

This API hook isn't used anywhere and most-likely exists because of the
general principle of C APIs, where if an API function does an
allocation/registration, it must also have an equivalent
deallocation/deregistration counterpart.

For devm_ functions this isn't all that true (for all cases), as the idea
of these function is to provide an auto-cleanup logic on drivers/system
de-init.

Removing this also discourages any weird logic that could be created with
such an API function.

Signed-off-by: Alexandru Ardelean <aardelean@deviqon.com>
Link: https://lore.kernel.org/r/20210625122324.327585-3-aardelean@deviqon.com
Signed-off-by: Mark Brown <broonie@kernel.org>