i2c: mux: reg: Remove class-based device auto-detection support

Legacy class-based device auto-detection shouldn't be used in new code.
Therefore remove support in i2c-mux-reg as long as we don't have a
user of this feature yet.

Link: https://lore.kernel.org/linux-i2c/a22978a4-88e4-46f4-b71c-032b22321599@gmail.com/
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>