_exports.h: drop creating dummy i2c entries and fixup config dependency

_exports.h: drop creating dummy i2c entries and fixup config dependency

There's really no good reason to create stub entries that would call a
function that doesn't even return anything sensible.

The existence of these two i2c_* functions depends on
CONFIG_IS_ENABLED(SYS_I2C_LEGACY), which does depend on !DM_I2C, but
is not equivalent to it. They are probably rather hard to use unless
CMD_I2C and something in U-Boot has called "i2c dev foo" to set the
current i2c bus before calling the standalone app, so keep that
dependency.

Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>
Reviewed-by: Tom Rini <trini@konsulko.com>