i2c: Drop unimplemented slave functions

i2c: Drop unimplemented slave functions

Drop the function declarations for slave mode support of i2c adapters.
This was never implemented, and by the time it is I bet we will want
something different anyway.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>