regulator: Remove mtk-dvfsrc-regulator.c

regulator: Remove mtk-dvfsrc-regulator.c

This driver never worked, and never got even compiled, because it was
missing the DVFSRC driver entirely, including headers it relies on!

In preparation of a full refactoring of this driver, remove it.

The Makefile and Kconfig entries are retained, as those are reused
as-is for the refactored code.

Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patch.msgid.link/20240610085735.147134-6-angelogioacchino.delregno@collabora.com
Signed-off-by: Mark Brown <broonie@kernel.org>