i2c: Drop legacy muxing pseudo-drivers

The i2c-amd756-s4882 and i2c-nforce2-s4985 muxing pseudo-drivers were
written at a time when the i2c core did not support muxing. They are
essentially board-specific hacks. If we had to add support for these
boards today, we would implement it in a completely different way.

These Tyan server boards are 19 years old by now, so I very much doubt
any of these is still running today. So let's just drop this clumsy
code. If anyone really still needs this support and complains, I'll
rewrite it in a proper way on top of i2c-mux.

This also fixes the following warnings:
drivers/i2c/busses/i2c-amd756.c:286:20: warning: symbol 'amd756_smbus' was not declared. Should it be static?
drivers/i2c/busses/i2c-nforce2.c:123:20: warning: symbol 'nforce2_smbus' was not declared. Should it be static?

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Andi Shyti <andi.shyti@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>